Rotomolded articles comprising at least one layer comprising a metallocene-catalyzed polyethylene resin

ABSTRACT

The present invention also relates to a process for preparing said rotomolded article.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of PCT/EP2015/076254 filed Nov. 10, 2015, which claims priority from EP 14193021.4 filed Nov. 13, 2014, which are incorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to a rotomolded article, comprising at least one layer comprising at least one metallocene catalyzed polyethylene resin.

BACKGROUND OF THE INVENTION

The process of rotational molding, also called rotomolding, of plastic has been known since the 1940s for the preparation of hollow plastic articles.

The rotomolding process consists of adding a thermoplastic polymer into a mold, rotating the mold so that all the points of the internal surface of the mold are in contact with the polymer while heating the mold, so as to deposit the aforementioned molten polymer on the internal surface of the mold. Thereafter, a stage of cooling allows the solidification of the plastic article, which is then removed from the mold.

Rotational molding is advantageous because it avoids applying stress and strain to the plastic, which generally occurs in other transformations, for example in injection molding. Indeed, the plastic does not undergo malaxation or compaction as in an extruder or in injection molding. Rotational molding is particularly suitable for preparing large-sized articles, such as furniture, tanks, drums, reservoirs etc.

Polyethylene represents more than 80% of the polymers used in the rotomolding markets. This is due to the outstanding resistance of polyethylene to thermal degradation during processing, to its easy grinding, good flowability and low temperature impact properties.

Rotomolding is used for the manufacture of simple to complex, hollow plastic products. A variety of materials can be used such as polyethylene, polycarbonate polyamide or polyvinylchloride (PVC). For example, polyethylene compositions are used for the production of a wide variety of articles. Particularly, polyethylene drums and tanks can easily be manufactured by rotomolding. Often these drums and tanks are exposed to numerous stresses during their lifetime, and that exposure may result in cracks or breaks. Thus, there is an ongoing need to develop rotomolded articles that display resistance to the development of cracks or breaks.

It is therefore an object of the present invention to provide rotomolded articles comprising polyethylene resin having improved mechanical properties and improved processability.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention provides a rotomolded article,

-   at least one metallocene-catalyzed polyethylene resin comprising at     least two metallocene-catalyzed polyethylene fractions A and B; and -   at least one ionomer; -   wherein the at least one metallocene-catalyzed polyethylene resin     comprises -   at least 25% to at most 55% by weight of polyethylene fraction A     based on the total weight of the at least one metallocene-catalyzed     polyethylene resin, wherein fraction A has a density at least 0.005     g/cm³ higher than the density of the at least one     metallocene-catalyzed polyethylene resin; and -   wherein the at least one metallocene-catalyzed polyethylene resin     has a density of at least 0.930 g/cm³ to at most 0.954 g/cm³ as     measured according to ASTM D-1505 at 23° C.; a melt index MI2 of at     least 1.0 g/10 min 10 min to at most 25.0 g/10 min 10 min as     determined according to ISO 1133, condition D, at 190° C. and under     a load of 2.16 kg.

The present invention also relates to a rotomolding process for preparing a rotomolded article according to the first aspect of the invention, comprising the steps of a) providing at least one metallocene-catalyzed polyethylene resin, said resin comprising at least two polyethylene fractions A and B, the resin comprising: at least 25% to at most 55% by weight of polyethylene fraction A based on the total weight of the at least one metallocene-catalyzed polyethylene resin, wherein fraction A has a density at least 0.005 g/cm³ higher than the density of the at least one metallocene-catalyzed polyethylene resin; wherein the at least one metallocene-catalyzed polyethylene resin has a density of at least 0.930 g/cm³ to at most 0.954 g/cm³ as measured according to ASTM D-1505 at 23° C.; a melt index MI2 of at least 1.0 g/10 min 10 min to at most 25.0 g/10 mini 0 min as determined according to ISO 1133, condition D, at 190° C. and under a load of 2.16 kg; b) compounding said at least one metallocene-catalyzed polyethylene resin with at least one ionomer, and c) rotomolding said at least one metallocene-catalyzed polyethylene resin compounded with said at least one ionomer, into at least one layer of an article.

The independent and dependent claims set out particular and preferred features of the invention. Features from the dependent claims may be combined with features of the independent or other dependent claims as appropriate.

The present invention will now be further described. In the following passages, different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 represents a graph plotting a TREF (temperature rising elution fractionation) profile (dW/dT (%/° C.)) as a function of temperature for polyethylene resin 1 according to an embodiment of the present invention.

FIG. 2 represents a graph plotting a TREF (temperature rising elution fractionation) profile (dW/dT (%/° C.)) as a function of temperature for polyethylene resin 2 according to an embodiment of the present invention.

FIG. 3 represents a rheometric dynamic analysis (“RDA”) graph plotting viscosity as a function of shear rate for four different polyethylene resins according to embodiments of the present invention, and a comparative example.

FIG. 4 represents a rheometric dynamic analysis (“RDA”) graph plotting storage modulus (G′) as a function of shear rate for four different polyethylene resins according to embodiments of the present invention, and a comparative example.

FIG. 5 represents a graph plotting the percentage of total picture area covered by bubbles (Aa, in %) as a function of the infra-red temperature of the sample (Tir, in ° C.) for rotomolded articles prepared with a polyethylene resin according to embodiments of the present invention, and a comparative example.

FIG. 6 represents a graph plotting the % of 3D shrinkage of rotomolded bottles comprising either a polyethylene resins according to embodiments of the invention or a comparative resin.

FIG. 7 represents a graph potting the level of shrinkage of rotomolded bottles comprising either polyethylene resins according to embodiments of the invention or a comparative resin.

FIG. 8 represents a graph plotting the break energy (J) and the total energy (J) at −20° C. and −40° C., of rotomolded bottles comprising either a polyethylene resins according to embodiments of the invention or a comparative resin.

FIG. 9 represents a graph plotting the maximal Von Mises stress (MPa) at the external surface of the tank in function of the thickness of the tank; for a tank comprising either a polyethylene resin according to the invention, or comparative resins. The horizontal dash lines represent the threshold for each resin.

FIG. 10 represents a graph plotting the maximal Von Mises stress (MPa) at the internal surface of the tank in function of the thickness of the tank; for a tank comprising either a polyethylene resin according to the invention, or comparative resins. The horizontal dash lines represent the threshold for each resin.

FIG. 11 represents a graph plotting the radial maximal displacement (MPa) of the tank in function of the thickness of the tank; for a tank comprising either a polyethylene resin according to the invention, or comparative resins.

DETAILED DESCRIPTION OF THE INVENTION

When describing the invention, the terms used are to be construed in accordance with the following definitions, unless a context dictates otherwise.

As used herein, the singular forms “a”, “an”, and “the” include both singular and plural referents unless the context clearly dictates otherwise. By way of example, “a resin” means one resin or more than one resin.

The terms “comprising”, “comprises” and “comprised of” as used herein are synonymous with “including”, “includes” or “containing”, “contains”, and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. It will be appreciated that the terms “comprising”, “comprises” and “comprised of” as used herein comprise the terms “consisting of”, “consists” and “consists of”.

The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 1, 2, 3, 4 when referring to, for example, a number of elements, and can also include 1.5, 2, 2.75 and 3.80, when referring to, for example, measurements). The recitation of end points also includes the end point values themselves (e.g. from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein.

All references cited in the present specification are hereby incorporated by reference in their entirety. In particular, the teachings of all references herein specifically referred to are incorporated by reference.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art.

Preferred statements (features) and embodiments of the articles, resins and uses of this invention are set herein below. Each statements and embodiments of the invention so defined may be combined with any other statement and/or embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features or statements indicated as being preferred or advantageous. Hereto, the present invention is in particular captured by any one or any combination of one or more of the below numbered aspects and embodiments 1 to 33, with any other statement and/or embodiments.

-   1. A rotomolded article, comprising at least one layer: said at     least one layer comprising:     -   at least one metallocene-catalyzed polyethylene resin comprising         at least two metallocene-catalyzed polyethylene fractions A and         B; and     -   at least one ionomer;     -   wherein the at least one metallocene-catalyzed polyethylene         resin comprises     -   at least 25% to at most 55% by weight of polyethylene fraction A         based on the total weight of the at least one         metallocene-catalyzed polyethylene resin, wherein fraction A has         a density at least 0.005 g/cm³ higher than the density of the at         least one metallocene-catalyzed polyethylene resin; and     -   wherein the at least one metallocene-catalyzed polyethylene         resin has a density of at least 0.930 g/cm³ to at most 0.954         g/cm³ as measured according to ASTM D-1505 at 23° C.; a melt         index MI2 of at least 1.0 g/10 min 10 min to at most 25.0 g/10         min 10 min as determined according to ISO 1133, condition D, at         190° C. and under a load of 2.16 kg. -   2. A rotomolded article, comprising at least one layer comprising:     -   at least one metallocene-catalyzed polyethylene resin comprising         at least two metallocene-catalyzed polyethylene fractions A and         B; and     -   at least one ionomer;     -   wherein the at least one metallocene-catalyzed polyethylene         resin comprises         -   at least 45% to at most 75% by weight of polyethylene             fraction B based on the total weight of the at least one             metallocene-catalyzed polyethylene resin, wherein fraction B             has a density at least 0.005 g/cm³ lower than the density of             the at least one metallocene-catalyzed polyethylene resin;             and         -   wherein the at least one metallocene-catalyzed polyethylene             resin has a density of at least 0.930 g/cm³ to at most 0.954             g/cm³ as measured according to ASTM D-1505 at 23° C.; a melt             index MI2 of at least 1.0 g/10 min to at most 25.0 g/10 min             as determined according to ISO 1133, condition D, at 190° C.             and under a load of 2.16 kg. -   3. A rotomolded article, comprising at least one     metallocene-catalyzed polyethylene resin comprising at least two     metallocene-catalyzed polyethylene fractions A and B; and at least     one ionomer compounded with said at least one polyethylene resin;     -   wherein the at least one metallocene-catalyzed polyethylene         resin comprises:     -   at least 25% to at most 55% by weight of polyethylene fraction A         based on the total weight of the at least one         metallocene-catalyzed polyethylene resin, wherein fraction A has         a density at least 0.005 g/cm³ higher than the density of the at         least one metallocene-catalyzed polyethylene resin; and     -   wherein the at least one metallocene-catalyzed polyethylene         resin has a density of at least 0.930 g/cm³ to at most 0.954         g/cm³ as measured according to ASTM D-1505 at 23° C.; a melt         index MI2 of at least 1.0 g/10 min to at most 25.0 g/10 min as         determined according to ISO 1133, condition D, at 190° C. and         under a load of 2.16 kg. -   4. The rotomolded article according to any one of statements 1 to 3,     wherein the at least one metallocene-catalyzed polyethylene resin     has a density of at least 0.9340 g/cm³ to at most 0.954 g/cm³ as     measured according to ASTM D-1505 at 23° C.; a melt index MI2 of at     least 2.5 g/10 min to at most 25.0 g/10 min as determined according     to ISO 1133, condition D, at 190° C. and under a load of 2.16 kg. -   5. The rotomolded article according to any one of statements 1 to 4,     wherein the at least one metallocene-catalyzed polyethylene resin     has a molecular weight distribution M_(w)/M_(n) of at least 2.0 to     at most 5.0, with M_(w) being the weight-average molecular weight     and M_(n) being the number-average molecular weight, preferably the     at least one metallocene-catalyzed polyethylene resin has a     molecular weight distribution M_(w)/M_(n) of at least 2.5 to at most     4.2, preferably of at least 2.6 to at most 4.2, preferably of at     least 2.6 to at most 4.0, preferably of at least 2.7 to at most 3.9,     preferably of at least 2.9 to at most 3.8, preferably of at least     2.9 to at most 3.6. -   6. The rotomolded article according to any one of statements 1 to 5,     wherein the Temperature Rising Elution Fractionation (TREF)     distribution curve of the at least one metallocene-catalyzed     polyethylene resin comprises at least one peak appearing at a     temperature of at least 97° C. to at most 105° C. and having an area     under the curve of at least 5.0% to at most 25.0%; as determined     using a crystallization rate of −0.5° C./min from 95° C. to 35° C.,     and column load volume of 1.90 ml, preferably as determined using     the conditions listed in Table 2. -   7. The rotomolded article according to statement 6, wherein the     Temperature Rising Elution Fractionation (TREF) distribution curve     of the at least one metallocene-catalyzed polyethylene resin further     comprises two more peaks having the following features:     -   peak 1 appearing at a temperature of at least 87.0° C. to at         most 93.0° C. and having an area under the curve of at least         25.0% to at most 50.0%;     -   peak 2 appearing at a temperature of at least 94.0° C. to at         most 98.0° C. and having an area under the curve of at least         35.0% to at most 60.0%. -   8. The rotomolded article according to any one of statements 6 or 7,     wherein the Temperature Rising Elution Fractionation (TREF)     distribution curve of the at least one metallocene-catalyzed     polyethylene resin has an area under the curve of at most 5.0% at a     temperature of 65° C. or below, preferably at most 4.0%, preferably     at most 3.0%. -   9. The rotomolded article according to any one of statements 6 to 8,     wherein at most 5.0% by weight of said at least one     metallocene-catalyzed polyethylene resin elutes in TREF at a     temperature of 65° C. or below, preferably at most 4.0% by weight,     preferably at most 3.0% by weight calculated based on the total     weight of crystallisable polyethylene fractions, as determined using     a crystallization rate of −0.5° C./min from 95° C. to 35° C., and     column load volume of 1.90 ml. -   10. The rotomolded article according to any one of statements 1 to     9, wherein the at least one metallocene-catalyzed polyethylene resin     has an HLMI of at least 85.0 g/10 min, preferably of at least 90.0     g/10 min, preferably of at least 100.0 g/10 min, preferably of at     least 105.0 g/10 min, as measured according to ISO 1133:1997     condition G at 190° C. and under a load of 21.6 kg. -   11. The rotomolded article according to any one of statements 1 to     10, wherein fraction A of the at least one metallocene-catalyzed     polyethylene resin has a density at least 0.010 g/cm³ higher than     the density of the at least one metallocene-catalyzed polyethylene     resin, preferably at least 0.015 g/cm³ higher, preferably at least     0.020 g/cm³ higher than the density of the at least one     metallocene-catalyzed polyethylene resin. -   12. The rotomolded article according to any one of statements 1 to     11, wherein fraction A of the at least one metallocene-catalyzed     polyethylene resin has a density as measured on the fluff of at     least 0.950 g/cm³, preferably of at least 0.955 g/cm³, preferably of     at least 0.958 g/cm³, preferably of at least 0.960 g/cm³. -   13. The rotomolded article according to any one of statements 1 to     12, wherein polyethylene fraction A of the at least one     metallocene-catalyzed polyethylene resin has a melt index MI2 as     measured on the fluff of at least 15.0 g/10 min, preferably at least     20.0 g/10 min, preferably at least 30.0 g/10 min, preferably of at     least 35.0 g/10 min, preferably of at least 40.0 g/10 min,     preferably of at least 45.0 g/10 min. -   14. The rotomolded article according to any one of statements 1 to     13, wherein the at least one metallocene-catalyzed polyethylene     resin comprises from 45% to at most 75% by weight of polyethylene     fraction B based on the total weight of the at least one     metallocene-catalyzed polyethylene resin, preferably from 50.0% to     at most 75.0% by weight, preferably from 55.0% to at most 70.0% by     weight, preferably from 56.0% to at most 65.0% by weight, preferably     from 58.0% to at most 60.0% by weight. -   15. The rotomolded article according to any one of statements 1 to     14, wherein fraction B of the at least one metallocene-catalyzed     polyethylene resin has a density of at least 0.005 g/cm³ lower than     the density of the at least one metallocene-catalyzed polyethylene     resin, preferably at least 0.010 g/cm³ lower, preferably at least     0.014 g/cm³ lower than the density of the polyethylene resin. -   16. The rotomolded article according to any one of statements 1 to     15, wherein fraction B of the at least one metallocene-catalyzed     polyethylene resin has a density of at most 0.935 g/cm³, preferably     at most 0.933 g/cm³. -   17. The rotomolded article according to any one of statements 1 to     16, wherein fraction B of the at least one metallocene-catalyzed     polyethylene resin has a density of at least 0.915 g/cm³, preferably     at least 0.918 g/cm³, preferably at least 0.920 g/cm³, preferably at     least 0.925 g/cm³. -   18. The rotomolded article according to any one of statements 1 to     17, wherein the at least one metallocene-catalyzed polyethylene     resin has a multimodal molecular weight distribution, and preferably     wherein said at least one metallocene-catalyzed polyethylene resin     has a bimodal molecular weight distribution. -   19. The rotomolded article according to any one of statements 1 to     18, wherein the at least one metallocene-catalyzed polyethylene     resin has a density of at least 0.938 g/cm³ to at most 0.950 g/cm³,     preferably of at least 0.939 g/cm³ to at most 0.950 g/cm³,     preferably of at least 0.940 g/cm³ to at most 0.949 g/cm³. -   20. The rotomolded article according to any one of statements 1 to     19, wherein the at least one metallocene-catalyzed polyethylene     resin has a melt index MI2 of preferably of at least 2.5 g/10 min to     at most 20.0 g/10 min, preferably of at least 2.5 g/10 min to at     most 15.0 g/10 min, preferably of at least 2.5 g/10 min to at most     11.0 g/10 min, preferably at least 2.5 g/10 min to at most 10.0 g/10     min, preferably at least 2.5 g/10 min to at most 8.0 g/10 min,     preferably at least 2.5 g/10 min to at most 6.0 g/10 min. -   21. The rotomolded article according to any one of statements 1 to     20, wherein the at least one metallocene-catalyzed polyethylene     resin has a rheology long chain branching index g_(rheo) of at most     0.99, for example at most 0.98, for example at most 0.97, for     example at most 0.96. -   22. The rotomolded article according to any one of statements 1 to     21, wherein each fraction of said at least one metallocene-catalyzed     polyethylene resin is prepared in different reactors of at least two     reactors connected in series, preferably at least two loop reactors     connected in series, preferably at least two slurry loop reactors     connected in series. -   23. The rotomolded article according to any one of statements 1 to     22, wherein the at least one metallocene-catalyzed polyethylene     resin is prepared using a process comprising the steps of:     -   (a) feeding ethylene monomer, a diluent, at least one         metallocene catalyst, optionally hydrogen, and optionally one or         more olefin co-monomers into at least one first slurry loop         reactor; polymerizing the ethylene monomer, and the optionally         one or more olefin co-monomers, in the presence of the         metallocene catalyst, and optional hydrogen, in said first         slurry loop reactor to produce a first polyethylene fraction A;         and     -   (b) feeding the first polyethylene fraction A to a second slurry         loop reactor serially connected to the first slurry loop         reactor, and in the second slurry loop reactor polymerizing         ethylene, and optionally one or more olefin co-monomers, in the         presence of the first polyethylene fraction, and optionally         hydrogen, thereby producing the metallocene         catalyzed-polyethylene resin. -   24. The rotomolded article according to any one of statements 1 to     23, wherein said at least one metallocene-catalyzed polyethylene     resin is prepared using a process comprising the steps of:     -   (a) feeding ethylene monomer, a diluent, at least one         metallocene catalyst, optionally hydrogen, into at least one         first slurry loop reactor; polymerizing the ethylene monomer, in         the presence of the metallocene catalyst, and optional hydrogen,         in said first slurry loop reactor to produce a first         polyethylene fraction A; and     -   (b) feeding the first polyethylene fraction A to a second slurry         loop reactor serially connected to the first slurry loop         reactor, and in the second slurry loop reactor polymerizing         ethylene, and one or more olefin co-monomers, in the presence of         the first polyethylene fraction, and optionally hydrogen,         thereby producing the metallocene catalyzed-polyethylene resin. -   25. The rotomolded article according to any one of statements 1 to     24, wherein fraction A of said at least one metallocene-catalyzed     polyethylene resin is an ethylene homopolymer and fraction B of said     at least one metallocene-catalyzed polyethylene resin is an ethylene     copolymer. -   26. The rotomolded article according to any one of statements 1 to     25, wherein at least said metallocene-catalyzed polyethylene resin     is compounded with at least one additional additive. -   27. The rotomolded article according to any one of statements 1 to     26, wherein said at least one ionomer is     -   a copolymer comprising one or more olefin or styrene monomers,         and one or more monomers comprising a functional group selected         from the group comprising carboxylate, sulfonate, and a         carboxylic acid anhydride; or     -   a grafted polyolefin or polystyrene comprising one or more         functional groups selected from the group comprising         carboxylate, sulfonate, a carboxylic acid anhydride,         phosphonate, and phosphinate. -   28. The rotomolded article according to any one of statements 1 to     27, wherein said at least one ionomer is a copolymer comprising     -   from about 50 wt % to about 99.8 wt % of one or more monomers         selected from the group comprising ethylene, propylene,         1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene,         2-methylpropene, styrene, 2-methylstyrene, 3-methylstyrene,         4-methylstyrene, alpha-methylstyrene, cis and/or trans         beta-methylstryrene, halogenated styrene isomers, vinyl         chloride, vinylidene chloride, butadiene, acrylonitrile,         methacrylonitrile, cyclopentene, and cyclohexene; copolymerized         with:     -   from about 0.2 wt % to about 30 wt % of one or more monomers         selected from the group comprising methacrylic acid, acrylic         acid, maleic acid or a monoester thereof, maleic anhydride,         ethacrylic acid, fumaric acid or a monoester thereof, crotonic         acid, itaconic acid or a monoester thereof, itaconic anhydride,         vinyl sulfonic acid, 2-methacryloyloxy-ethanesulfonate,         styrenesulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid         (AMPS), vinylphosphonic acid, 2-(methacryloyloxy)ethylphosphate,         mesaconic acid, citraconic acid or a monoester thereof,         glutaconic acid or a monoester thereof, methylmaleic acid or a         monoester thereof, methylmaleic anhydride, citraconic anhydride,         glutaconic anhydride,         endobicyclo-[2,2,1]-5-heptene-2,3-dicarboxylic acid or a         monoester thereof, and         endobicyclo-[2,2,1]-5-heptene-2,3-dicarboxylic acid anhydride. -   29. The rotomolded article according to any one of statements 1 to     28, wherein said at least one ionomer is a polymer selected from the     group comprising polyethylene, polypropylene, a     polyethylene/polypropylene copolymer, polybutene, polyhexene,     polycyclopentene, polycyclohexene, and polystyrene, said polymer     being grafted with one or more compounds selected from the group     comprising maleic anhydride, acrylic acid or a salt thereof, and     methacrylic acid or a salt thereof. -   30. The rotomolded article according to any one of statements 1 to     29, wherein said at least one metallocene-catalyzed polyethylene     resin is compounded with 0.1% to 20% by weight of at least one     ionomer, preferably with 0.2% to 10% by weight of at least one     ionomer, preferably with 0.5% to 7% by weight, preferably from 0.5%     to 5% by weight, more preferably from 1.0% to 3.0% by weight     relative to the total weight of said layer. -   31. The rotomolded article according to any one of statements 1 to     30, wherein the article is a tank, a drum, a container, a bin, a     vat, a jerrycan, a can, a cistern, a bottle, boat or part thereof,     float, buoy, structural part. -   32. A rotomolding process for preparing a rotomolded article     according to any one of statements 1 to 31, comprising the steps     of a) providing at least one metallocene-catalyzed polyethylene     resin comprising at least two polyethylene fractions A and B,     wherein the at least one metallocene-catalyzed polyethylene resin     comprises: at least 25% to at most 55% by weight of polyethylene     fraction A based on the total weight of the at least one     metallocene-catalyzed polyethylene resin, wherein fraction A has a     density at least 0.005 g/cm³ higher than the density of the at least     one metallocene-catalyzed polyethylene resin; wherein the     polyethylene resin has a density of at least 0.930 g/cm³ to at most     0.954 g/cm³ as measured according to ASTM D-1505 at 23° C.; a melt     index MI2 of at least 1.0 g/10 min to at most 25.0 g/10 min as     determined according to ISO 1133, condition D, at 190° C. and under     a load of 2.16 kg; b) compounding said at least one     metallocene-catalyzed polyethylene resin with at least one ionomer,     and c) rotomolding said at least one metallocene-catalyzed     polyethylene resin compounded with said at least one ionomer, into     at least one layer of an article. -   33. The rotomolding process according to statement 32, wherein said     at least one metallocene-catalyzed polyethylene resin is prepared in     at least two reactors connected in series.

The present invention relates to a rotomolded article, comprising at least one layer, wherein said at least one layer comprises at least one metallocene-catalyzed polyethylene resin comprising at least two metallocene-catalyzed polyethylene fractions A and B; and at least one ionomer;

-   wherein the polyethylene resin comprises: at least 25% to at most     55% by weight of polyethylene fraction A based on the total weight     of the polyethylene resin, wherein fraction A has a density at least     0.005 g/cm³ higher than the density of the polyethylene resin; -   wherein the polyethylene resin has a density of at least 0.930 g/cm³     to at most 0.954 g/cm³ as measured according to ASTM D-1505 at 23°     C.; a melt index MI2 of at least 1.0 g/10 min to at most 25.0 g/10     min as determined according to ISO 1133, condition D, at 190° C. and     under a load of 2.16 kg.

The term “polyethylene resin” as used herein refers to the polyethylene fluff or powder that is extruded, and/or melted and/or pelleted and can be produced through compounding and homogenizing of the polyethylene resin as taught herein, for instance, with mixing and/or extruder equipment.

The term “fluff” or “powder” as used herein refers to the polyethylene material with the hard catalyst particle at the core of each grain and is defined as the polymer material after it exits the polymerization reactor (or final polymerization reactor in the case of multiple reactors connected in series).

In some embodiments, fraction A has a density as measured on the polymer material after it exits the polymerization reactor of at least 0.005 g/cm³ higher than the density of the polyethylene resin, preferably at least 0.010 g/cm³ higher, preferably at least 0.015 g/cm³ higher, preferably at least 0.020 g/cm³ higher than the density of the at least one metallocene-catalyzed polyethylene resin.

In some embodiments, fraction A has a density as measured on the polymer material after it exits the polymerization reactor of at least 0.950 g/cm³; preferably of at least 0.955 g/cm³, preferably of at least 0.958 g/cm³, preferably of at least 0.960 g/cm³.

In some embodiments, fraction A has a melt index MI2 as measured on the polymer material after it exits the polymerization reactor of at least 15.0 g/10 min, preferably at least 20.0 g/10 min, preferably at least 25.0 g/10 min, preferably at least 30.0 g/10 min, preferably of at least 35.0 g/10 min, preferably of at least 40.0 g/10 min, preferably of at least 45.0 g/10 min.

In a preferred embodiment, the at least one metallocene-catalyzed polyethylene resin suitable for use in the rotomolded article has a multimodal molecular weight distribution. In some embodiments, the metallocene-catalyzed polyethylene resin has a bimodal molecular weight distribution.

As used herein, the term “monomodal polyethylene” or “polyethylene with a monomodal molecular weight distribution” refers to polyethylene having one maximum in their molecular weight distribution curve, which is also defined as a unimodal distribution curve. As used herein, the term “polyethylene with a bimodal molecular weight distribution” or “bimodal polyethylene” it is meant, polyethylene having a distribution curve being the sum of two unimodal molecular weight distribution curves, and refers to a polyethylene product having two distinct but possibly overlapping populations of polyethylene macromolecules each having different weight average molecular weights. By the term “polyethylenes with a multimodal molecular weight distribution” or “multimodal polyethylenes” it is meant polyethylenes with a distribution curve being the sum of at least two, preferably more than two unimodal distribution curves, and refers to a polyethylene product having two or more distinct but possibly overlapping populations of polyethylene macromolecules each having different weight average molecular weights. The multimodal polyethylene resin can have an “apparent monomodal” molecular weight distribution, which is a molecular weight distribution curve with a single peak and no shoulder. Nevertheless, the polyethylene resin will still be multimodal if it comprises two distinct populations of polyethylene macromolecules each having a different weight average molecular weights, as defined above, for example when the two distinct populations were prepared in different reactors and/or under different conditions.

In some embodiment, the at least one metallocene-catalyzed polyethylene resin suitable for use in the rotomolded article has a multimodal, preferably bimodal, molecular weight distribution. The at least one metallocene-catalyzed polyethylene resin for use in the rotomolded article can be produced by polymerizing ethylene and one or more optional comonomers, optionally hydrogen, in the presence of a metallocene catalyst system.

As used herein, the term “catalyst” refers to a substance that causes a change in the rate of a polymerization reaction. In the present invention, it is especially applicable to catalysts suitable for the polymerization of ethylene to polyethylene. The present invention especially relates to polyethylene prepared in the presence of metallocene catalyst. As used herein, the terms “metallocene-catalyzed polyethylene resin”, and “metallocene-catalyzed polyethylene” are synonymous and used interchangeably and refers to a polyethylene prepared in the presence of a metallocene catalyst.

The term “metallocene catalyst” is used herein to describe any transition metal complexes comprising metal atoms bonded to one or more ligands. The metallocene catalysts are compounds of Group IV transition metals of the Periodic Table such as titanium, zirconium, hafnium, etc., and have a coordinated structure with a metal compound and ligands composed of one or two groups of cyclopentadienyl, indenyl, fluorenyl or their derivatives. The structure and geometry of the metallocene can be varied to adapt to the specific need of the producer depending on the desired polymer. Metallocenes comprise a single metal site, which allows for more control of branching and molecular weight distribution of the polymer. Monomers are inserted between the metal and the growing chain of polymer.

In one embodiment of the present invention, the metallocene catalyst is a compound of formula (I) or (II) (Ar)₂MQ₂  (I); or R″(Ar)₂MQ₂  (II),

-   wherein the metallocenes according to formula (I) are non-bridged     metallocenes and the metallocenes according to formula (II) are     bridged metallocenes; -   wherein said metallocene according to formula (I) or (II) has two Ar     bound to M which can be the same or different from each other; -   wherein Ar is an aromatic ring, group or moiety and wherein each Ar     is independently selected from the group consisting of     cyclopentadienyl, indenyl (IND), tetrahydroindenyl (THI), and     fluorenyl, wherein each of said groups may be optionally substituted     with one or more substituents each independently selected from the     group consisting of halogen, hydrosilyl, SiR₃ wherein R is a     hydrocarbyl having 1 to 20 carbon atoms, and a hydrocarbyl having 1     to 20 carbon atoms, and wherein said hydrocarbyl optionally contains     one or more atoms selected from the group comprising B, Si, S, O, F,     Cl, and P; -   wherein M is a transition metal selected from the group consisting     of titanium, zirconium, hafnium, and vanadium; and preferably is     zirconium; -   wherein each Q is independently selected from the group consisting     of halogen; a hydrocarboxy having 1 to 20 carbon atoms; and a     hydrocarbyl having 1 to 20 carbon atoms and wherein said hydrocarbyl     optionally contains one or more atoms selected from the group     comprising B, Si, S, O, F, Cl, and P; and -   wherein R″ is a divalent group or moiety bridging the two Ar groups     and selected from the group consisting of C₁-C₂₀ alkylene,     germanium, silicon, siloxane, alkylphosphine, and an amine, and     wherein said R″ is optionally substituted with one or more     substituents each independently selected from the group consisting     of halogen, hydrosilyl, SiR₃ wherein R is a hydrocarbyl having 1 to     20 carbon atoms, and a hydrocarbyl having 1 to 20 carbon atoms and     wherein said hydrocarbyl optionally contains one or more atoms     selected from the group comprising B, Si, S, O, F, Cl, and P.

Preferably, the metallocene comprises a bridged bis-indenyl and/or a bridged bis-tetrahydrogenated indenyl component. In some embodiments, the metallocene can be selected from one of the following formula (IVa) or (IVb):

wherein each R in formula (IVa) or (IVb) is the same or different and is selected independently from hydrogen or XR′_(v) in which X is chosen from Group 14 of the Periodic Table (preferably carbon), oxygen or nitrogen and each R′ is the same or different and is chosen from hydrogen or a hydrocarbyl of from 1 to 20 carbon atoms and v+1 is the valence of X, preferably R is a hydrogen, methyl, ethyl, n-propyl, iso-propyl, n-butyl, tert-butyl group; R″ is a structural bridge between the two indenyl or tetrahydrogenated indenyls that comprises a C₁-C₄ alkylene radical, a dialkyl germanium, silicon or siloxane, or an alkyl phosphine or amine radical; Q is a hydrocarbyl radical having from 1 to 20 carbon atoms or a halogen, preferably Q is F, CI or Br; and M is a transition metal Group 4 of the Periodic Table or vanadium.

Each indenyl or tetrahydro indenyl component may be substituted with R in the same way or differently from one another at one or more positions of either of the fused rings. Each substituent is independently chosen.

If the cyclopentadienyl ring is substituted, its substituent groups must not be so bulky so as to affect coordination of the olefin monomer to the metal M. Any substituents XR′_(y) on the cyclopentadienyl ring are preferably methyl. More preferably, at least one and most preferably both cyclopentadienyl rings are unsubstituted.

In a particularly preferred embodiment, the metallocene comprises a bridged unsubstituted bis-indenyl and/or bis-tetrahydrogenated indenyl i.e. all R are hydrogens. More preferably, the metallocene comprises a bridged unsubstituted bis-tetrahydrogenated indenyl.

Illustrative examples of metallocene catalysts comprise but are not limited to bis(cyclopentadienyl) zirconium dichloride (Cp₂ZrCl₂), bis(cyclopentadienyl) titanium dichloride (Cp₂TiCl₂), bis(cyclopentadienyl) hafnium dichloride (Cp₂HfCl₂); bis(tetrahydroindenyl) zirconium dichloride, bis(indenyl) zirconium dichloride, and bis(n-butyl-cyclopentadienyl) zirconium dichloride; ethylenebis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride, ethylenebis(1-indenyl) zirconium dichloride, dimethylsilylene bis(2-methyl-4-phenyl-inden-1-yl) zirconium dichloride, diphenylmethylene (cyclopentadienyl)(fluoren-9-yl) zirconium dichloride, and dimethylmethylene [1-(4-tert-butyl-2-methyl-cyclopentadienyl)](fluoren-9-yl) zirconium dichloride. Most preferably the metallocene is ethylene-bis(tetrahydroindenyl)zirconium dichloride or ethylene-bis(tetrahydroindenyl) zirconium difluoride.

As used herein, the term “hydrocarbyl having 1 to 20 carbon atoms” refers to a moiety selected from the group comprising a linear or branched C₁-C₂₀ alkyl; C₃-C₂₀ cycloalkyl; C₆-C₂₀ aryl; C₇-C₂₀ alkylaryl and C₇-C₂₀ arylalkyl, or any combinations thereof. Exemplary hydrocarbyl groups are methyl, ethyl, propyl, butyl, amyl, isoamyl, hexyl, isobutyl, heptyl, octyl, nonyl, decyl, cetyl, 2-ethylhexyl, and phenyl.

As used herein, the term “hydrocarboxy having 1 to 20 carbon atoms” refers to a moiety with the formula hydrocarbyl-O—, wherein the hydrocarbyl has 1 to 20 carbon atoms as described herein. Preferred hydrocarboxy groups are selected from the group comprising alkyloxy, alkenyloxy, cycloalkyloxy or aralkoxy groups.

As used herein, the term “alkyl”, by itself or as part of another substituent, refers to straight or branched saturated hydrocarbon group joined by single carbon-carbon bonds having 1 or more carbon atom, for example 1 to 12 carbon atoms, for example 1 to 6 carbon atoms, for example 1 to 4 carbon atoms. When a subscript is used herein following a carbon atom, the subscript refers to the number of carbon atoms that the named group may contain. Thus, for example, C₁₋₁₂alkyl means an alkyl of 1 to 12 carbon atoms. Examples of alkyl groups are methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, 2-methylbutyl, pentyl and its chain isomers, hexyl and its chain isomers, heptyl and its chain isomers, octyl and its chain isomers, nonyl and its chain isomers, decyl and its chain isomers, undecyl and its chain isomers, dodecyl and its chain isomers. Alkyl groups have the general formula C_(n)H_(2n+1).

As used herein, the term “cycloalkyl”, by itself or as part of another substituent, refers to a saturated or partially saturated cyclic alkyl radical. Cycloalkyl groups have the general formula C_(n)H_(2n−1). When a subscript is used herein following a carbon atom, the subscript refers to the number of carbon atoms that the named group may contain. Thus, examples of C₃₋₆cycloalkyl include cyclopropyl, cyclobutyl, cyclopentyl, or cyclohexyl.

As used herein, the term “aryl”, by itself or as part of another substituent, refers to a radical derived from an aromatic ring, such as phenyl, naphthyl, indanyl, or 1,2,3,4-tetrahydro-naphthyl. When a subscript is used herein following a carbon atom, the subscript refers to the number of carbon atoms that the named group may contain.

As used herein, the term “alkylaryl”, by itself or as part of another substituent, refers to refers to an aryl group as defined herein, wherein a hydrogen atom is replaced by an alkyl as defined herein. When a subscript is used herein following a carbon atom, the subscript refers to the number of carbon atoms that the named group or subgroup may contain.

As used herein, the term “arylalkyl”, by itself or as part of another substituent, refers to refers to an alkyl group as defined herein, wherein a hydrogen atom is replaced by a aryl as defined herein. When a subscript is used herein following a carbon atom, the subscript refers to the number of carbon atoms that the named group may contain. Examples of C₆₋₁₀arylC₁₋₆alkyl radicals include benzyl, phenethyl, dibenzylmethyl, methylphenylmethyl, 3-(2-naphthyl)-butyl, and the like.

As used herein, the term “alkylene”, by itself or as part of another substituent, refers to alkyl groups that are divalent, i.e., with two single bonds for attachment to two other groups. Alkylene groups may be linear or branched and may be substituted as indicated herein. Non-limiting examples of alkylene groups include methylene (—CH₂—), ethylene (—CH₂—CH₂—), methylmethylene (—CH(CH₃)—), 1-methyl-ethylene (—CH(CH₃)—CH₂—), n-propylene (—CH₂—CH₂—CH₂—), 2-methylpropylene (—CH₂—CH(CH₃)—CH₂—), 3-methylpropylene (—CH₂—CH₂—CH(CH₃)—), n-butylene (—CH₂—CH₂—CH₂—CH₂—), 2-methylbutylene (—CH₂—CH(CH₃)—CH₂—CH₂—), 4-methylbutylene (—CH₂—CH₂—CH₂—CH(CH₃)—), pentylene and its chain isomers, hexylene and its chain isomers, heptylene and its chain isomers, octylene and its chain isomers, nonylene and its chain isomers, decylene and its chain isomers, undecylene and its chain isomers, dodecylene and its chain isomers. When a subscript is used herein following a carbon atom, the subscript refers to the number of carbon atoms that the named group may contain. For example, C₁-C₂₀ alkylene refers to an alkylene having between 1 and 20 carbon atoms.

Exemplary halogen atoms include chlorine, bromine, fluorine and iodine, wherein fluorine and chlorine are preferred.

The metallocene catalysts used herein are preferably provided on a solid support. The support can be an inert organic or inorganic solid, which is chemically unreactive with any of the components of the conventional metallocene catalyst. Suitable support materials for the supported catalyst include solid inorganic oxides, such as silica, alumina, magnesium oxide, titanium oxide, thorium oxide, as well as mixed oxides of silica and one or more Group 2 or 13 metal oxides, such as silica-magnesia and silica-alumina mixed oxides. Silica, alumina, and mixed oxides of silica and one or more Group 2 or 13 metal oxides are preferred support materials. Preferred examples of such mixed oxides are the silica-aluminas. Most preferred is a silica compound. In a preferred embodiment, the metallocene catalyst is provided on a solid support, preferably a silica support. The silica may be in granular, agglomerated, fumed or other form.

In some embodiments, the support of the metallocene catalyst is a porous support, and preferably a porous silica support having a surface area comprised between 200 m²/g and 900 m²/g. In another embodiment, the support of the polymerization catalyst is a porous support, and preferably a porous silica support having an average pore volume comprised between 0.5 ml/g and 4 ml/g. In yet another embodiment, the support of the polymerization catalyst is a porous support, and preferably a porous silica support having an average pore diameter comprised between 50 Å and 300 Å, and preferably between 75 Å and 220 Å.

Preferably, the supported metallocene catalyst is activated. The cocatalyst, which activates the metallocene catalyst component, can be any cocatalyst known for this purpose such as an aluminium-containing cocatalyst, a boron-containing cocatalyst or a fluorinated catalyst. The aluminium-containing cocatalyst may comprise an alumoxane, an alkyl aluminium, a Lewis acid and/or a fluorinated catalytic support.

In some embodiments, alumoxane is used as an activating agent for the metallocene catalyst. The alumoxane can be used in conjunction with a catalyst in order to improve the activity of the catalyst during the polymerization reaction.

As used herein, the term “alumoxane” and “aluminoxane” are used interchangeably, and refer to a substance, which is capable of activating the metallocene catalyst. In some embodiments, alumoxanes comprise oligomeric linear and/or cyclic alkyl alumoxanes. In a further embodiment, the alumoxane has formula (V) or (VI) R^(a)—(Al(R^(a))—O)_(x)-AlR^(a) ₂  (V) for oligomeric, linear alumoxanes; or (—Al(R^(a))—O—)_(y)  (VI) for oligomeric, cyclic alumoxanes

-   wherein x is 1-40, and preferably 10-20; -   wherein y is 3-40, and preferably 3-20; and -   wherein each R^(a) is independently selected from a C₁-C₈alkyl, and     preferably is methyl. In a preferred embodiment, the alumoxane is     methylalumoxane (MAO).

In a preferred embodiment, the metallocene catalyst is a supported metallocene-alumoxane catalyst comprising a metallocene and an alumoxane which are bound on a porous silica support. Preferably, the metallocene catalyst is a bridged bis-indenyl catalyst and/or a bridged bis-tetrahydrogenated indenyl catalyst.

One or more aluminiumalkyl represented by the formula AlR^(b) _(x) can be used as additional co-catalyst, wherein each R^(b) is the same or different and is selected from halogens or from alkoxy or alkyl groups having from 1 to 12 carbon atoms and x is from 1 to 3. Non-limiting examples are Tri-Ethyl Aluminum (TEAL), Tri-Iso-Butyl Aluminum (TIBAL), Tri-Methyl Aluminum (TMA), and Methyl-Methyl-Ethyl Aluminum (MMEAL). Especially suitable are trialkylaluminiums, the most preferred being triisobutylaluminium (TIBAL) and triethylaluminum (TEAL).

Suitable ethylene polymerization includes but is not limited to homopolymerization of ethylene, or copolymerization of ethylene and a higher 1-olefin co-monomer.

As used herein, the term “co-monomer” refers to olefin co-monomers which are suitable for being polymerized with alpha-olefin monomer. Co-monomers may comprise but are not limited to aliphatic C3-C20 alpha-olefins. Examples of suitable aliphatic C3-C20 alpha-olefins include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, and 1-eicosene. In some embodiments of the present invention, said co-monomer is 1-hexene.

In a preferred embodiment, fraction A of the at least one metallocene-catalyzed polyethylene resin for use in said rotomolded article is an ethylene homopolymer. In a preferred embodiment, fraction B of the at least one metallocene-catalyzed polyethylene resin for use in said rotomolded article is an ethylene copolymer. The term “ethylene copolymer” as used herein is intended to encompass polymers which consist essentially of repeat units deriving from ethylene and at least one other C3-C20 alpha olefin co-monomer, preferably the co-monomer is 1-hexene. The term “ethylene homopolymer” as used herein is intended to encompass polymers which consist essentially of repeat units deriving from ethylene. Homopolymers may, for example, comprise at least 99.8% preferably 99.9% by weight of repeats units derived from ethylene.

Preferably, the at least one metallocene-catalyzed polyethylene resin for use in said rotomolded article is prepared in two or more serially connected reactors. In some embodiments, the at least one metallocene-catalyzed polyethylene resin comprises two metallocene-catalyzed polyethylene fractions A and B, wherein each fraction is prepared in different reactors of two reactors connected in series.

The metallocene-catalyzed polyethylene resin is preferably obtained by operating the at least two reactors under different polymerization conditions.

The metallocene-catalyzed polyethylene resin can be prepared out in gas, solution or slurry phase. Slurry polymerization is preferably used to prepare the polyethylene resin composition, preferably in a slurry loop reactor or a continuously stirred reactor.

Preferably, the metallocene-catalyzed polyethylene resin is prepared in two or more serially connected reactors, comprising at least one first and at least one second reactors, preferably loop reactors, more preferably slurry loop reactors, most preferably liquid full loop reactors in the presence of same or different metallocene catalysts. The most preferred polymerization process is carried out in two serially connected slurry loop reactors, advantageously liquid full loop reactors i.e. a double loop reactor.

As used herein, the terms “loop reactor” and “slurry loop reactor” may be used interchangeably herein.

The catalyst is preferably added to the loop reactor as catalyst slurry. As used herein, the term “catalyst slurry” refers to a composition comprising catalyst solid particles and a diluent. The solid particles can be suspended in the diluent, either spontaneously or by homogenization techniques, such as mixing. The solid particles can be non-homogeneously distributed in a diluent and form sediment or deposit.

As used herein, the term “diluent” refers to any organic diluent, which does not dissolve the synthesized polyolefin. As used herein, the term “diluent” refers to diluents in a liquid state, liquid at room temperature and preferably liquid under the pressure conditions in the loop reactor. Suitable diluents comprise but are not limited to hydrocarbon diluents such as aliphatic, cycloaliphatic and aromatic hydrocarbon solvents, or halogenated versions of such solvents. Preferred solvents are C12 or lower, straight chain or branched chain, saturated hydrocarbons, C5 to C9 saturated alicyclic or aromatic hydrocarbons or C2 to C6 halogenated hydrocarbons. Non-limiting illustrative examples of solvents are butane, isobutane, pentane, hexane, heptane, cyclopentane, cyclohexane, cycloheptane, methyl cyclopentane, methyl cyclohexane, isooctane, benzene, toluene, xylene, chloroform, chlorobenzenes, tetrachloroethylene, dichloroethane and trichloroethane, preferably isobutane.

In certain embodiments, each loop reactor may comprise interconnected pipes, defining a reactor path. In certain embodiments, each loop reactor may comprise at least two vertical pipes, at least one upper segment of reactor piping, at least one lower segment of reactor piping, joined end to end by junctions to form a complete loop, one or more feed lines, one or more outlets, one or more cooling jackets per pipe, and one pump, thus defining a continuous flow path for a polymer slurry. The vertical sections of the pipe segments are preferably provided with cooling jackets. Polymerization heat can be extracted by means of cooling water circulating in these jackets of the reactor. The loop reactor preferably operates in a liquid full mode.

In certain embodiments, the first and second loop reactors may be connected through means such as a transfer line or one or more settling legs. In some embodiments, the first polyethylene fraction may be transferred from the first loop reactor to the second loop reactor through a transfer line. In some embodiments, the first polyethylene fraction may be discharged in batches, sequentially or continuously from the first loop reactor through one or more settling legs, and transferred to the second loop reactor via a transfer line.

In some embodiments, the at least one metallocene-catalyzed polyethylene resin for use in the rotomolded article, is prepared using a process comprising the steps of:

-   (a) feeding ethylene monomer, a diluent, at least one metallocene     catalyst, optionally hydrogen, and optionally one or more olefin     co-monomers into at least one first slurry loop reactor;     polymerizing the ethylene monomer, and the optionally one or more     olefin co-monomers, in the presence of the metallocene catalyst, and     optional hydrogen, in said first slurry loop reactor to produce a     first polyethylene fraction; and -   (b) feeding the first polyethylene fraction to a second slurry loop     reactor serially connected to the first slurry loop reactor, and in     the second slurry loop reactor polymerizing ethylene, and optionally     one or more olefin co-monomers, in the presence of the first     polyethylene fraction, and optionally hydrogen, thereby producing     the metallocene catalyzed-polyethylene resin.

The polymerization steps can be performed over a wide temperature range. In certain embodiments, the polymerization steps may be performed at a temperature from 20° C. to 125° C., preferably from 60° C. to 110° C., more preferably from 75° C. to 100° C. and most preferably from 78° C. to 98° C. Preferably, the temperature range may be within the range from 75° C. to 100° C. and most preferably from 78° C. to 98° C.

In certain embodiments, the polymerization steps may be performed at a pressure from about 20 bar to about 100 bar, preferably from about 30 bar to about 50 bar, and more preferably from about 37 bar to about 45 bar.

In some embodiments, reactants comprise the monomer ethylene, isobutane as hydrocarbon diluent, a supported metallocene catalyst, and optionally at least one co-monomer such as 1-hexene are used.

In some embodiments, the rotomolded article comprises at least one metallocene-catalyzed polyethylene resin comprises two metallocene-catalyzed polyethylene fractions A and B, and comprises:

-   at least 35% to at most 50% by weight of polyethylene fraction A     based on the total weight of the polyethylene resin, wherein     fraction A has a density at least 0.010 g/cm³ higher than the     density of the polyethylene resin; -   and the at least one metallocene-catalyzed polyethylene resin has a     density of at least 0.930 g/cm³ to at most 0.954 g/cm³ as measured     according to ASTM D-1505 at 23° C.; a melt index MI2 of at least 2.5     g/10 min to at most 25.0 g/10 min. Preferably said at least one     metallocene-catalyzed polyethylene resin has a molecular weight     distribution M_(w)/M_(n) of at least 2.6 to at most 4.2, with M_(w)     being the weight-average molecular weight and M_(n) being the     number-average molecular weight.

In a preferred embodiment, the rotomolded article comprises at least one ionomer compounded with at least one metallocene-catalyzed polyethylene resin comprising two polyethylene fractions A and B, fraction B having a higher molecular weight and a lower density than fraction A, with each fraction being prepared in different reactors of two slurry loop reactors connected in series. The molecular weight of the fractions prepared in each of the reactors can be adjusted by known techniques such as varying the amount of hydrogen used. The density of the fractions prepared in each of the reactors can be adjusted by known techniques such as varying the amount of co-monomer used.

In a preferred embodiment, fraction A has a density at least 0.005 g/cm³ higher than the density of the polyethylene resin, preferably at least 0.010 g/cm³ higher, preferably at least 0.015 g/cm³ higher, more preferably at least 0.020 g/cm³ higher than the density of the polyethylene resin.

In a preferred embodiment, polyethylene fraction A has a density as measured on the fluff of at least 0.950 g/cm³, preferably of at least 0.955 g/cm³, preferably of at least 0.960 g/cm³, preferably of at least 0.965 g/cm³.

In a preferred embodiment, polyethylene fraction A has a melt index MI2 as measured on the fluff of at least 15.0 g/10 min to at most 110.0 g/10 min, preferably of at least 20.0 g/10 min to at most 110 g/10 min, preferably of at least 25.0 g/10 min to at most 110 g/10 min, preferably of at least 30.0 g/10 min. to at most 110.0 g/10 min, preferably of at least 35.0 g/10 min to at most 105 g/10 min, preferably of at least 40 g/10 min to at most 100 g/10 min.

In an embodiment, fraction A is present in an amount ranging from at least 25% to at most 55% by weight based on the total weight of polyethylene resin; preferably ranging from 30% to 50%, for example from 35% to 45%, preferably from 38% to 42%, by weight based on the total weight of polyethylene resin.

In some embodiments, the metallocene-catalyzed polyethylene resin according to the present invention has a density of at least 0.934 g/cm³ to at most 0.954 g/cm³, preferably at least 0.938 g/cm³ to at most 0.950 g/cm³, preferably of at least 0.939 g/cm³ to at most 0.9490 g/cm³, preferably of at least 0.940 g/cm³ to at most 0.949 g/cm³.

In some embodiments, the metallocene-catalyzed polyethylene resin has a melt index MI2 of at least 3.0 g/10 min to at most 20.0 g/10 min, preferably the polyethylene resin has a melt index MI2 of at least 3.5 g/10 min to at most 15.0 g/10 min, preferably of at least 3.5 g/10 min to at most 11.0 g/10 min, preferably at least 3.5 g/10 min to at most 10.0 g/10 min, preferably at least 3.5 g/10 min to at most 8.0 g/10 min, preferably at least 3.5 g/10 min to at most 6.0 g/10 min. Under normal production conditions in a production plant it is expected that the melt index MI2 will be higher for the fluff.

In some embodiments, the metallocene-catalyzed polyethylene resin has an HLMI of at least 85.0 g/10 min to at most 200.0 g/10 min as measured according to ISO 1133:1997 condition G at 190° C. and under a load of 21.6 kg. Preferably the polyethylene resin has an HLMI of at least 90.0 g/10 min to at most 180.0 g/10 min, preferably of at least 95.0 g/10 min to at most 180.0 g/10 min, preferably of at least 100 g/10 min to at most 170.0 g/10 min, preferably of at least 100 g/10 min to at most 160.0 g/10 min, preferably of at least 100.0 g/10 min to at most 150.0 g/10 min.

In some embodiments, the metallocene-catalyzed polyethylene resin has a molecular weight distribution M_(w)/M_(n) of at least 2.6 to at most 4.2. Preferably the metallocene-catalyzed polyethylene resin according to the present invention has a molecular weight distribution M_(w)/M_(n) of at least 2.7 to at most 4.0, preferably of at least 2.8 to at most 3.8, preferably of at least 2.9 to at most 3.5.

As used herein, the molecular weight (M_(n) (number average molecular weight), M_(w) (weight average molecular weight) and molecular weight distributions D (M_(w)/M_(n)), and D′ (M_(Z)/M_(W)) were determined by size exclusion chromatography (SEC) and in particular by gel permeation chromatography (GPC).

In some embodiments, the metallocene-catalyzed polyethylene resin has a rheology long chain branching index g_(rheo) of at most 0.99, for example at most 0.98, for example at most 0.97, for example at most 0.96. As used herein the long chain branching (LCB) index g_(rheo) can be obtained by rheology according to the formula, as described in the example section.

In some embodiments, the metallocene-catalyzed polyethylene resin has a rheology long chain branching index g_(rheo) of at least 0.70, for example at least 0.74, for example at least 0.78, for example at least 0.80, for example at least 0.84, for example at least 0.88, for example at least 0.90.

In some embodiments, the metallocene-catalyzed polyethylene resin has a rheology long chain branching index g_(rheo) of at least 0.70 to at most 0.99. Preferably the polyethylene resin has a rheology long chain branching index g_(rheo) at least 0.74 to at most 0.98; preferably of at least 0.78 to at most 0.97; preferably of at least 0.80 to at most 0.96, preferably of at least 0.85 to at most 0.92.

In some embodiments, the at least one layer of the rotomolded article comprises at least one metallocene-catalyzed polyethylene resin comprising from at least 45% to at most 75% by weight of polyethylene fraction B based on the total weight of the polyethylene resin. Preferably the metallocene-catalyzed polyethylene resin comprises from at least 50% to at most 75% by weight of polyethylene fraction B based on the total weight of the polyethylene resin, preferably from at least 55% to at most 70%, preferably from at least 56% to at most 65%, preferably from at least 58% to at most 62%.

In some embodiments, the metallocene-catalyzed polyethylene resin according to the present invention has a Temperature Rising Elution Fractionation (TREF) distribution curve of the resin comprises at least one peak appearing at a temperature of at least 97.0° C. to at most 105.0° C. and having an area under the curve of at least 5.0% to at most 25.0%; as determined using a crystallization rate of −0.5° C./min from 95° C. to 35° C., and column load volume of 1.90 ml. Preferably, the metallocene-catalyzed polyethylene resin according to the present invention has a Temperature Rising Elution Fractionation (TREF) distribution curve of the resin comprises at least one peak appearing at a temperature of at least 99.0° C. to at most 103.0° C. and having an area under the curve of at least 5.0% to at most 25.0%; as determined using a crystallization rate of −0.5° C./min from 95° C. to 35° C., and column load volume of 1.90 ml. Preferably, the metallocene-catalyzed polyethylene resin according to the present invention has a Temperature Rising Elution Fractionation (TREF) distribution curve of the resin comprises at least one peak appearing at a temperature of at least 100.0° C. to at most 102.0° C. and having an area under the curve of at least 5.0% to at most 25.0%; as determined using a crystallization rate of −0.5° C./min from 95° C. to 35° C., and column load volume of 1.90 ml.

Preferably, the Temperature Rising Elution Fractionation (TREF) distribution curve of the resin further comprises two more peaks having the following features:

-   peak 1 appearing at a temperature of at least 87.0° C. to at most     93.0° C. and having an area under the curve of at least 25.0% to at     most 50.0%; -   peak 2 appearing at a temperature of at least 94.0° C. to at most     98.0° C. and having an area under the curve of at least 35.0% to at     most 60.0%.

In some embodiments, the at least one metallocene-catalyzed polyethylene resin for use in the rotomolded article has a Temperature Rising Elution Fractionation (TREF) distribution curve characterized by three peaks having the following features:

-   peak 1 appearing at a temperature of at least 87.0° C. to at most     93.0° C. and having an area under the curve of at least 25.0% to at     most 50.0%; -   peak 2 appearing at a temperature of at least 94.0° C. to at most     98.0° C. and having an area under the curve of at least 35.0% to at     most 60.0%; -   peak 3 appearing at a temperature of at least 99.0° C. to at most     103.0° C. and having an area under the curve of at least 5.0% to at     most 25.0%.

In some embodiments, the metallocene-catalyzed polyethylene resin according to the present invention has a Temperature Rising Elution Fractionation (TREF) distribution curve characterized by three peaks having the following features:

-   peak 1 appearing at a temperature of at least 87.0° C. to at most     93.0° C. and having an area under the curve of at least 25.0% to at     most 50.0%; -   peak 2 appearing at a temperature of at least 94.0° C. to at most     98.0° C. and having an area under the curve of at least 35.0% to at     most 60.0%; -   peak 3 appearing at a temperature of at least 100.0° C. to at most     102.0° C. and having an area under the curve of at least 5.0% to at     most 25.0%.

For metallocene-catalyzed polyethylene resin comprising two fractions A and B, wherein fraction B is prepared in the second reactor, the density of fraction B is linked to that of the density of fraction A by the following expression: d=W _(A) *d _(A)+(1−W _(A))*d _(B) wherein d is the density of the final polyethylene fluff, W_(A) is the weight fraction of fraction A, d_(A) is the density of fraction A, d_(B) is the density of fraction B, and wherein the sum of both fractions A and B by weight (W_(A)+W_(B)) is 1. A similar formula can be expressed for any polyethylene resin comprising more than two fractions.

In some embodiments, the metallocene-catalyzed polyethylene resin comprises a polyethylene fraction B having a density of at least 0.915 g/cm³ to at most 0.935 g/cm³.

For metallocene-catalyzed polyethylene resin comprising two fractions A and B, the HLMI of fraction B is linked to that of the HLMI of fraction A by the following expression: Log HLMI^(final) =W _(A)×Log HLMI_(A) +W _(B)×Log HLMI_(B) wherein Log HLMI^(final) is the Log HLMI of the final polyethylene fluff, W_(A) is the weight fraction of fraction A, Log HLMI_(A) is the Log HLMI of fraction A, Log HLMI_(B) is the Log HLMI of fraction B, and wherein the sum of both fractions A and B by weight (W_(A)+W_(B)) is 1. A similar formula can be expressed for any polyethylene resin comprising more than two fractions.

For metallocene-catalyzed polyethylene resin comprising two fractions A and B, the MI2 of fraction B is linked to that of the MI2 of fraction A by the following expression: Log MI2^(final) =W _(A)×Log MI2_(A) +W _(B)×Log MI2_(B) wherein Log MI2^(final) is the Log MI2 of the final polyethylene fluff, W_(A) is the weight fraction of fraction A, Log MI2_(A) is the Log MI2 of fraction A, Log MI2_(B) is the Log MI2 of fraction B, and wherein the sum of both fractions A and B by weight (W_(A)+W_(B)) is 1. A similar formula can be expressed for any multimodal polyethylene resin comprising more than two fractions.

In some embodiments, the metallocene-catalyzed polyethylene resin comprises a fraction B having a melt index MI2 of at most 5.0 g/10 min, preferably at most 3.0 g/10 min, preferably at most 2.5 g/10 min, preferably at most 2.0 g/10 min, preferably at least 0.10 g/10 min, preferably at least 0.5 g/10 min.

According to the invention the at least one polyethylene resin is compounded with at least one ionomer.

Suitable ionomers include polyelectrolyte that comprises copolymers containing both electrically neutral repeating units and a fraction of ionized or ionizable units. They link in such a way that, even though they are stiff at room temperature, the bonds may be broken down thermally and the new linkages will cause the material to act as though it were a thermoplastic material.

In particular embodiments, the ionomer can be:

-   -   a copolymer comprising one or more olefin monomers (such as         ethylene, propylene, 1-butene, etc.) or styrene monomers (such         as styrene, 2-methylstyrene, 3-methylstyrene, etc.), and one or         more monomers comprising a functional group selected from the         group comprising carboxylate, sulfonate, and a carboxylic acid         anhydride; or     -   a grafted polyolefin or polystyrene comprising one or more         functional groups selected from the group comprising         carboxylate, sulfonate, a carboxylic acid anhydride,         phosphonate, and phosphinate; preferably carboxylate, sulfonate,         or a carboxylic acid anhydride, most preferably carboxylate.

In certain embodiments, the ionomer is a copolymer comprising:

-   -   one or more monomers selected from the group comprising         ethylene, propylene, 1-butene, 1-pentene, 4-methyl-1-pentene,         1-hexene, 2-methylpropene, styrene, 2-methylstyrene,         3-methylstyrene, 4-methylstyrene, alpha-methylstyrene, cis         and/or trans beta-methylstryrene, halogenated styrene isomers,         vinyl chloride, vinylidene chloride, butadiene, acrylonitrile,         methacrylonitrile, cyclopentene, and cyclohexene; copolymerized         with     -   one or more monomers selected from the group comprising         methacrylic acid, acrylic acid, maleic acid or a monoester         thereof, maleic anhydride, ethacrylic acid, fumaric acid or a         monoester thereof, crotonic acid, itaconic acid or a monoester         thereof, itaconic anhydride, vinyl sulfonic acid,         2-methacryloyloxy-ethanesulfonate, styrenesulfonic acid,         2-acrylamido-2-methylpropane sulfonic acid (AMPS),         vinylphosphonic acid, 2-(methacryloyloxy)ethylphosphate,         mesaconic acid, citraconic acid or a monoester thereof,         glutaconic acid or a monoester thereof, methylmaleic acid or a         monoester thereof, methylmaleic anhydride, citraconic anhydride,         glutaconic anhydride,         endobicyclo-[2,2,1]-5-heptene-2,3-dicarboxylic acid or a         monoester thereof, and         endobicyclo-[2,2,1]-5-heptene-2,3-dicarboxylic acid anhydride.

In certain embodiments, the ionomer is a copolymer comprising:

-   -   from about 50 wt % to about 99.8 wt % of one or more monomers         selected from the group comprising ethylene, propylene,         1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene,         2-methylpropene, styrene, 2-methylstyrene, 3-methylstyrene,         4-methylstyrene, alpha-methylstyrene, cis and/or trans         beta-methylstryrene, halogenated styrene isomers, vinyl         chloride, vinylidene chloride, butadiene, acrylonitrile,         methacrylonitrile, cyclopentene, and cyclohexene; copolymerized         with     -   from about 0.2 wt % to about 30 wt % of one or more monomers         selected from the group comprising methacrylic acid, acrylic         acid, maleic acid or a monoester thereof, maleic anhydride,         ethacrylic acid, fumaric acid or a monoester thereof, crotonic         acid, itaconic acid or a monoester thereof, itaconic anhydride,         vinyl sulfonic acid, 2-methacryloyloxy-ethanesulfonate,         styrenesulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid         (AMPS), vinylphosphonic acid, 2-(methacryloyloxy)ethylphosphate,         mesaconic acid, citraconic acid or a monoester thereof,         glutaconic acid or a monoester thereof, methylmaleic acid or a         monoester thereof, methylmaleic anhydride, citraconic anhydride,         glutaconic anhydride,         endobicyclo-[2,2,1]-5-heptene-2,3-dicarboxylic acid or a         monoester thereof, and         endobicyclo-[2,2,1]-5-heptene-2,3-dicarboxylic acid anhydride.

In particular embodiments, the ionomer is a polymer selected from the group comprising polyethylene, polypropylene, a polyethylene/polypropylene copolymer, polybutene, polyhexene, polycyclopentene, polycyclohexene, and polystyrene, said polymer being grafted with one or more compounds selected from the group comprising maleic anhydride, acrylic acid or a salt thereof, and methacrylic acid or a salt thereof.

Suitable ionomers can be prepared by methods well known in the art such as, for example, as described in U.S. Pat. Nos. 3,264,272, 3,404,134, and 3,355,319, each of which is hereby incorporated by reference. Suitable commercially available ionomers include, but are not limited to, those known under the tradenames of Surlyn® (DuPont), EEA Copolymer (ethylene-ethyl acrylate copolymer), Hycar® (Goodrich), Iotek® (ExxonMobil), Priex® (Solvay Engineered Polymers), AClyn® (Honeywell International), Nafion® (DuPont), and Thionic® (Uniroyal).

In an embodiment the at least one resin is compounded with 0.1% to 20% by weight of at least one ionomer, preferably 0.2% to 10% by weight of at least one ionomer, preferably from 0.5% to 7% by weight, preferably from 0.5% to 5% by weight, more preferably from 1.0% to 3.0% by weight relative to the total weight of said compounded resin.

The polyethylene resin may be compounded with one or more additional additives, in particular additives such as, by way of example, processing aids, mold-release agents, anti-slip agents, primary and secondary antioxidants, light stabilizers, anti-UV agents, acid scavengers, flame retardants, fillers, nanocomposites, lubricants, antistatic additives, nucleating/clarifying agents, antibacterial agents, plastisizers, colorants/pigments/dyes, sealant resins and mixtures thereof. Illustrative pigments or colorants include titanium dioxide, carbon black, cobalt aluminum oxides such as cobalt blue, and chromium oxides such as chromium oxide green. Pigments such as ultramarine blue, phthalocyanine blue and iron oxide red are also suitable. Specific examples of additives include lubricants and mold-release agents such as calcium stearate, zinc stearate, SHT, antioxidants such as Irgafos®168, Irganox®1010, and Irganox®1076, anti-slip agents such as erucamide, light stabilizers such as Tinuvin®622, Tinuvin®326 and Cyasorb THT®4611, and nucleating agents such as Milliken HPN20E™.

The articles obtained by rotomolding are generally hollow parts without any welding lines, such as tanks, drums, containers, bins, vats, jerrycans, cans, cisterns, boxes, bumpers, furniture (bath tubs), signs and ballards, planters, playground slides, car parts such as car doors, car bodies and car seats, airplane parts, nautical and aquatic equipment, buoys, floats, boards, planks and joints. In some embodiments, the rotomolded article is selected from the group comprising bottles, tanks, drums, containers, bins, vats, jerrycans, cans and cisterns, boat or parts thereof, and structural parts.

Rotomolded articles include single and multilayered constructions in the form of bottles, tanks, large hollow articles, rigid food containers and toys, for example.

The present invention also encompasses a rotomolding process for preparing a rotomolded article according to the invention, comprising the steps of a) providing at least one metallocene-catalyzed polyethylene resin as described herein; b) compounding said at least one resin with at least one ionomer; and b) rotomolding said compounded polyethylene resin into an article. The article can be a mono-layered article only having one layer, or it can be multilayered such as a bi-layered or tri-layered rotomolded article.

These articles can have one or more openings and/or inserts of plastic or metal and/or “kiss offs” which are reinforcing junctions or bridges between two surfaces within the article.

Rotational molding is a process well-known to the person skilled in the art. The various processes of rotational molding usually comprise the stages of a) loading of the mold; b) rotation of the mold; c) heating of the mold; d) cooling; and e) release from the mold. The mold can be made of any material known in the art for such a purpose. For example, the mold can be an aluminum mold or a Teflon mold. Teflon molds are preferred to avoid any sticking to the mold due to the decreased shrinkage and warpage according to the article of the invention. The mold may be then loaded with powder and/or micropellets comprising the at least one metallocene-catalyzed polyethylene resin as described herein. The quantity of powder and/or of micropellets introduced into the mold depends on the size of the article and on the desired wall thickness. In some embodiments, the wall thickness of the article is of at least 2 mm to at most 25 mm when the article comprises one or more layers, or at least 500 μm to at most 25 mm when the comprises a monolayer (single layer)

The rotation of the mold can be generally carried out around two perpendicular axes.

The heating step (c) of the mold occurs simultaneously with the rotation of the mold in step (b).

In some embodiments, the heating step can be carried out in an oven or by electric heating elements. In some other preferred embodiments heating can be carried out with a mold heated by an oil-filled heating jacket, as in for example, the Leonardo® rotomolding machine from Persico®. The heating temperature of the oven, electric heating elements or oil can vary from 150° C. to 350° C., while the temperature of the air in the interior of the blow mold (peak internal air temperature) can vary from 205° C. to 215° C. One generally uses a temperature of at least of 10° C. higher, preferentially at least of 20° C. higher, more preferentially at least of 30° C. higher compared to the melting point of the layer that one wishes to mold In another embodiment, heating can also be carried out by microwaves.

The duration of the molding varies according to dimensions and the thickness of rotomolded article; it can range from 5 minutes to 600 minutes.

The duration and the time of cooling step depends on the installation, on the dimensions of the article to be molded and of the type of article which one wishes to obtain. As mentioned previously, it is possible to cool the mold and/or the article contained in the mold. To cool the mold from the outside, one can use air at room temperature, water between 5° C. and 25° C. or oil between 5° C. and 80° C. To cool the article from the inside of the mold, one can inject air and/or inert gas such as nitrogen and/or spray water (like a mist) within the interior of the mold, for example at a temperature of 25° C. The time of cooling generally varies between 5 minutes and 500 minutes depending on the thickness of the rotomolded article and the process used for cooling. When the article has a thickness of more than 10 mm, the mold can preferably be cooled from both the inside of the mold and the outside, preferably using Ar or inert gas such as nitrogen or a spray of water (mist).

According to a mode of realization, the cooling of the mold and/or article obtained can be done in just one step until a temperature ranging between room temperature and a temperature lower than 100° C. is obtained.

Thereafter, the article can be released from the mold. Release of the article from the mold can be generally carried out when the article has sufficient rigidity. The release from the mold can be generally done at a temperature lower than 100° C.

According to another mode of realization, the cooling of the mold and/or article obtained can be done in just one step until a temperature ranging between room temperature and a temperature lower than 100° C. is obtained.

According to another mode of realization, the cooling of the mold and/or article comprises the following steps:

-   -   i. cooling until a temperature ranging between 100° C. and 150°         C., preferably between 100° C. and 130° C., is reached,     -   ii. maintaining this temperature for 1 minute to 60 minutes,     -   iii. cooling again until a temperature ranging between the room         temperature and a temperature lower than 100° C. is reached.

Thereafter, the article can be released from the mold. Release of the article from the mold can be generally carried out when the article has sufficient rigidity.

The rotational molding can be carried out under inert gas in the absence of oxygen. In order to do so, one can for example add into the mold a compound which liberates carbon dioxide, such as dry ice. This can be for example together with the powder or pellets of the different components. Dry ice generates carbon dioxide during the heating and rotating steps of the molding process. One can also purge the mold with an inert gas, such as nitrogen, by injecting nitrogen after closing the mold.

The walls of the articles can comprise one or more successive layers, at least one of which comprises a metallocene-catalyzed polyethylene resin as described herein. It is thus possible to manufacture articles with walls comprising for example two or more layers.

There are several known methods to manufacture multilayered rotomolded articles: by manual introduction of material during the rotomolding cycle, or by the use of a drop-box, or by a one-shot system wherein each layer has a different melting temperature and are introduced into the mold together.

In some embodiments, manual addition involves moving the mold from the oven, removing a vent tube or plug that creates an opening in the part and adding more material using a funnel or wand. This operation can be repeated for each additional layer.

In some embodiments, a drop-box typically comprises the material for a particular layer and it is an insulated container that holds material until it is released at the appropriate time during the cycle. The signal for release of material can be usually transmitted as a pressure pulse via the airline through the arm of the machine. The insulation can be kept cool to prevent the material inside the box from melting.

The present invention also discloses a method for rotomolding the articles according to the invention for one or more layers comprising the steps of (in no particular order) according to the process known to the skilled person:

-   -   a) feeding at least one metallocene-catalyzed polyethylene resin         as described herein for a first layer into a mold;     -   b) placing the filled mold in pre-heated oven;     -   c) rotating the filled mold about two perpendicular axes;     -   d) optionally feeding a composition for a second layer;     -   e) optionally feeding a composition for a third layer, followed         by repeating steps (b) and (c);     -   f) optionally feeding desired additional layers, each addition         followed by repeating steps (b) and (c).     -   g) cooling and retrieving the rotomolded article

Preferably, the process is carried out in the order described.

Articles obtained by the rotomolding process according to the invention do not have a point of weakness. They show homogeneous characteristics, such as for example a uniform thickness of the walls as well as very good surface aspects both internally and externally, without displaying any deformation, bubble or other defects. In addition, the resin for use in the process presents little or no sagging behavior during the rotomolding process. In particular, the articles obtained according to the present invention display low warpage, low sagging and low deformation. They also benefit from improved stiffness and rigidity; as well as good tensile properties.

The invention will now be illustrated by the following, non-limiting illustrations of particular embodiments of the invention.

EXAMPLES

Test Methods:

The density was measured according to the method of standard ASTM 1505 at a temperature of 23° C.

The melt index MI2 was measured according to the method of standard ISO 1133:1997, condition D, at 190° C. and under a load of 2.16 kg.

High load melt index HLMI was measured according to the method of standard ISO 1133:1997, condition G, at 190° C. and under a load of 21.6 kg.

The melt index MI5 was measured according to the method of standard ISO 1133:1997, condition T, at 190° C. and under a load of 5 kg.

The molecular weight (M_(n) (number average molecular weight), M_(w) (weight average molecular weight) and molecular weight distributions d (Mw/Mn), and d′ (Mz/Mw) were determined by size exclusion chromatography (SEC) and in particular by gel permeation chromatography (GPC). Briefly, a GPC-IR5 from Polymer Char was used: 10 mg polyethylene sample was dissolved at 160° C. in 10 ml of trichlorobenzene for 1 hour. Injection volume: about 400 μl, automatic sample preparation and injection temperature: 160° C. Column temperature: 145° C. Detector temperature: 160° C. Two Shodex AT-806MS (Showa Denko) and one Styragel HT6E (Waters) columns were used with a flow rate of 1 ml/min. Detector: Infrared detector (2800-3000 cm⁻¹). Calibration: narrow standards of polystyrene (PS) (commercially available). Calculation of molecular weight Mi of each fraction i of eluted polyethylene is based on the Mark-Houwink relation (log₁₀(M_(PE))=0.965909×log₁₀(M_(PS))−0.28264) (cut off on the low molecular weight end at M_(PE)=1000).

The molecular weight averages used in establishing molecular weight/property relationships are the number average (M_(n)), weight average (M_(w)) and z average (M_(Z)) molecular weight. These averages are defined by the following expressions and are determined form the calculated M_(i):

$M_{n} = {\frac{\sum\limits_{i}{N_{i}M_{i}}}{\sum\limits_{i}N_{i}} = {\frac{\sum\limits_{i}W_{i}}{\sum\limits_{i}{W_{i}/M_{i}}} = \frac{\sum\limits_{i}h_{i}}{\sum\limits_{i}{h_{i}/M_{i}}}}}$ $M_{w} = {\frac{\sum\limits_{i}{N_{i}M_{i}^{2}}}{\sum\limits_{i}{N_{i}M_{i}}} = {\frac{\sum\limits_{i}{W_{i}M_{i}}}{\sum\limits_{i}W_{i}} = \frac{\sum\limits_{i}{h_{i}M_{i}}}{\sum\limits_{i}h_{i}}}}$ $M_{z} = {\frac{\sum\limits_{i}{N_{i}M_{i}^{3}}}{\sum\limits_{i}{N_{i}M_{i}^{2}}} = {\frac{\sum\limits_{i}{W_{i}M_{i}^{2}}}{\sum\limits_{i}{W_{i}M_{i}}} = \frac{\sum\limits_{i}{h_{i}M_{i}^{2}}}{\sum\limits_{i}{h_{i}M_{i}}}}}$

Here N_(i) and W_(i) are the number and weight, respectively, of molecules having molecular weight Mi. The third representation in each case (farthest right) defines how one obtains these averages from SEC chromatograms. h_(i) is the height (from baseline) of the SEC curve at the i_(th) elution fraction and M_(i) is the molecular weight of species eluting at this increment.

Rheology long chain branching index g_(rheo) was measured according to the formula, as described in WO 2008/113680:

${g_{rheo}({PE})} = \frac{M_{w}({SEC})}{M_{w}\left( {\eta_{0},{MWD},{SCB}} \right)}$

-   wherein Mw (SEC) is the weight average molecular weight obtained     from size exclusion chromatography expressed in kDa; -   and wherein Mw (η₀, MWD, SCB) is determined according to the     following, also expressed in kDa:     M _(w)(η₀ ,MWD,SCB)=exp(1.7789+0.199769 LnM _(n)+0.209026(Ln η     ₀)+0.955(ln ρ)−0.007561(LnM _(z))(Ln η ₀)+0.02355(ln M _(z))²)     wherein the zero shear viscosity η0 in Pa·s is obtained from a     frequency sweep experiment combined with a creep experiment, in     order to extend the frequency range to values down to 10⁻⁴ s⁻¹ or     lower, and taking the usual assumption of equivalence of angular     frequency (rad/s) and shear rate; wherein zero shear viscosity η0 is     estimated by fitting with Carreau-Yasuda flow curve (η−W) at a     temperature of 190° C., obtained by oscillatory shear rheology on     ARES-G2 equipment (manufactured by TA Instruments) in the linear     viscoelasticity domain; wherein circular frequency (W in rad/s)     varies from 0.05-0.1 rad/s to 250-500 rad/s, typically 0.1 to 250     rad/s, and the shear strain is typically 10%. In practice, the creep     experiment is carried out at a temperature of 190° C. under nitrogen     atmosphere with a stress level such that after 1200 s the total     strain is less than 20%; wherein the apparatus used is an AR-G2     manufactured by TA instruments.

The TREF analysis was performed on a TREF model 200 series instrument equipped with Infrared detector from Polymer Char. The samples were dissolved in 1,2-dichlorobenzene at 150° C. for 1 h. The following parameters as shown in Table A were used.

TABLE A METHOD INFORMATION Dissolution Rate (° C./min) 40 Stabilization Rate (° C./min) 40 Crystallization Rate 1(° C./min) 0.5 Elution Rate (° C./min) 1 Cleaning rate (° C./min) 30 Dissolution temperature (° C.) 150 Stabilization temperature (° C.) 95 Crystallization temperature (° C.) 35 Elution init temp (° C.) 35 Elution temperature (° C.) 140 Post elution temperature (° C.) 150 Cleaning temperature (° C.) 150 Dissolution time (min) 60 Stabilization time (min) 45 Crystallization time (min) 10 Pre-injection time (min) 10 Soluble Fraction time (min) 10 post elution time (min) 10 High rpm 200 Low rpm 100 T on (s) 5 T off (s) 120 Dissolution stirring High Stabilization stirring High Filling vessels volume (ml) 20 Filling vessels pick up speed (ml/min) 40 Filling vessels pump speed (ml/min) 15 Analysis discarded sample volume (ml) 2 Analysis discarded waste volume (ml) 6 Analysis sample volume (ml) 0.3 Column load volume (ml) 1.9 Analysis waste volume (ml) 5 Analysis returned volume (ml) 1 Analysis pick up rate (ml/min) 8 Analysis dispensing rate (ml/min) 3 Cleaning volume (ml) 30 Cleaning pick up speed (ml/min) 40 Cleaning pump speed (ml/min) 15 Top oven temperature (° C.) 140 Pump Flow (ml/min) 0.5

Dynamic rheometry analyses (RDA) were performed on an ARES rheometer from TA Instruments (Waters SA), measured on parallel plates with a diameter of 25 mm. Temperature was 190° C., deformation was 10%, and the scanning frequency was from 0.1 to 300 rad/s.

ESCR was measured according to ASTM D 1693-method B at 50° C. using 10% or 100% Igepal CO-630 as a chemical agent (wherein Igepal CO-630 (CAS Number 68412-54-4) is commercially available from Rhodia), or ASTM D 1693-method A at 50° C. using 100% Igepal CO-630.

Tensile modulus was measured according to ISO 527-2.

Determination of the shrinkage/warpage of the molded object: The measurement of the rotomolded article compared to the mold itself was done by three-dimensional analysis according to the method described in international patent application published WO 2008/151988. In brief the method comprises the steps of: a) subjecting the external surface of the rotomolded part to three-dimensional analysis by optical digitization; b) subjecting the internal surface of the mold to three-dimensional analysis by optical digitization; c) keying the molded part into the mold; d) mapping the volume between mold and molded part point by point; e) creating a bank of maps using different resins; f) creating a bank of maps using different oven temperatures; g) creating a bank of maps using different cooling rates; h) selecting the parameters of steps i) and/or j) and/or i) that give the best balance of deformations. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional images taken around a single axis of rotation.—A fixed point of the molded part is keyed to the mold.

Example 1 Preparation of Resins

Polyethylene resins: four resins (Resins 1, 2, 3, and 4) having bimodal molecular weight distribution were prepared in two serially connected slurry loop reactors (double loop reactor) under the conditions given below in Table 1. The polyethylene resins (Resins 1, 2, 3, and 4) were prepared with dimethylated ethylene bis(tetrahydroindenyl) zirconium, methylalumoxane (MAO) supported catalyst. The silica support was sold by PQ Corporation under the name ES70W. It had the following specifications: specific surface area=291 m²/g; D50=40 μm; porous volume=1.63 ml/g; and span=1.62. The supported metallocene-MAO-catalyst comprised 30% in weight of MAO (sold by Albemarle) and 2% in weight of metallocene. Pelletization was performed on a ZSK58 twin screw extruder.

TABLE 1 operating conditions and analytical results for each of the resins Resin 1 Resin 2 Resin 3 Resin 4 First reactor Temperature (° C.) 90 90 90 90 operating ethylene (kg/h) 19.0 19.0 19.0 19.0 conditions 1-hexene (kg/h) 0.0 0.0 0.0 0.0 H₂ (Nl/h) 30.0 37.1 33.3 34.0 Isobutane (kg/h) 50 50 50 50 First reactor MI2 (g/10 min) 49.1 94.8 50.2 55.9 Analytical Density (g/cm³) 0.9664 0.9702 0.9721 0.9692 results CONTRIBUTION First reactor (wt %) 40.6 40.6 40.4 40.9 Second reactor Temperature (° C.) 83.0 83.0 83.0 83.0 Operating ethylene (kg/h) 27.0 27.0 27.0 27.0 conditions 1-hexene (Kg/h) 4.2 4.5 5.0 3.9 H₂ (Nl/h) 19.0 17.5 14.7 15.2 Isobutane (kg/h) 45 45 45 45 Second Reactor MI2 (g/10 min) 6.3 6.8 4.6 4.8 Analytical MI5 (g/10 min) 18.5 20.0 13.9 14.5 Results final HLMI (g/10 min) 165 190 111 120 fluff SR2 26 28 24 25 SR5 9 10 8 8 Density (g/cm³) 0.9449 0.9449 0.9454 0.9468 Density: D1 1^(st) reactor (g/cm³) 0.9664 0.9702 0.9721 0.9692 fraction made in 1st reactor Density: final (g/cm³) 0.9449 0.9449 0.9454 0.9468 fluff Calculated Fraction made (g/cm³) 0.9301 0.9276 0.9273 0.9313 density D2: in 2^(nd) reactor fraction made in 2^(nd) rector Delta density D1 − D2 (g/cm³) 0.0215 0.0253 0.0267 0.0224 Calculated MI2 Fraction made (g/10 min) 1.56 1.13 0.91 0.88 in 2^(nd) reactor PELLETS MI2 (g/10 min) 5.15 5.09 3.99 4.03 MI5 (g/10 min) 15.3 15.2 12.0 12.2 HLMI (g/10 min) 140 148 104 103 SR2 27 29 26 26 Density (g/cm³) 0.9438 0.9435 0.9448 0.9481 GPC Mn (Da) 17868 16943 18443 18523 FINAL PELLETS Mw (Da) 56815 58106 59836 58616 Mz (Da) 117088 124796 126250 118633 D 3.2 3.4 3.2 3.2 D′ 2.1 2.1 2.1 2.0 RDA ghreo 0.91 0.92 0.87 0.87 FINAL FLUFF SR/MWD 8.2 8.2 7.6 7.8 (~LCB)

Example 2 TREF Analysis

Resins 1 and 2 were fractionated according to their chemical compositions by a Temperature Rising Elution Fractionation (TREF) process. The results are shown in FIGS. 1 and 2. FIG. 1 shows the TREF distribution profile of Resin 1, while FIG. 2 shows the TREF distribution profile of Resin 2. Table 2 shows the percentages of the polyethylene resin 1, and polyethylene 2 that elutes at a given temperature, in % by weight.

TABLE 2 Temperature (° C.) Resin 1 (wt % eluted) Resin 2 (wt % eluted) 40.00 0.90 0.43 50.00 1.75 1.09 60.00 2.82 2.15 70.00 4.29 3.71 80.00 7.69 7.50 85.00 12.21 12.45 87.00 15.42 16.72 90.00 24.80 31.11 93.00 38.56 46.40 94.00 43.47 51.01 95.00 49.48 56.52 96.00 56.94 63.31 97.00 65.27 70.67 98.00 73.45 78.09 99.00 79.98 83.55 100.00 84.30 87.32 101.00 88.40 91.11 102.00 94.00 95.81 103.00 97.84 98.42 104.00 98.85 99.03 105.00 99.12 99.22 106.00 99.22 99.30 107.00 99.28 99.35 108.00 99.33 99.39 109.00 99.39 99.42 110.00 99.43 99.45 115.00 99.57 99.61 120.00 99.71 99.79 125.00 99.87 99.97 130.00 99.94 99.99 135.00 100.00 100.00

For each of the polymers tested, the temperature of each of the peaks observed in the TREF distribution curves, the percentage of the area under said peaks are displayed in Table 3.

TABLE 3 Peak 1 Peak 2 Peak 3 Temp Area Temp Area Temp Area ° C. (%) ° C. (%) ° C. (%) Resin 1 91.5 32.1 96.9 55.2 101.4 15.1 Resin 2 90.0 44.2 96.7 43.3 101.2 12.2

Example 3

Resins as prepared in Example 1 were compounded in a twin screw extruder (Werner & Pfleiderer ZSK 58). The temperature profile along the extruder barrel was 180° C., and the temperature of the die was 210° C. The screw speed was 200 rpm, the residence time of 0.5 min, and the number of passes was 1. The compounded resin compositions are shown in Table 4, wherein Surlyn®1650E (commercially available from Du Pont) Surlyn® 1650 E is a zinc ionomer, more particularly a copolymer of ethylene and methacrylic acid.

TABLE 4 P P P P P P 130 131 130b 131b 132 133 Resin 1 2 1 2 3 4 Surlyn ®1650E / / 1 1 1 1 weight weight weight weight % % % %

Compression molded samples of the compounded resins were analyzed.

ESCR and RDA analyses were performed and compared with the results obtained when using a commercially-available rotomolding-grade metallocene-catalyzed monomodal polyethylene resin Lumicene® mPE M4041 UV from Total Refining & Chemicals, having an MI2 (ISO 1133/D 2.16 kg-190° C.) of 4 g/10 min, and a density of 0.940 g/cm³ (ISO 1183). The results are shown in FIG. 3, which plots viscosity (Pa·s) of each resin as a function of shear rate (Rad/s); and in FIG. 4, which shows the plot of the storage modulus (G′) of each resin as a function of shear rate (Rad/s), and in Table 5

TABLE 5 M4041UV P 130 P 131 P 132 P 133 ESCR 10% Igepal CO <100 654 >1000 120 18 630 (in hours) ESCR 100% Igepal CO 70 >1000 630 (in hours) Tensile Modulus (MPa) 665 821 826 782 841

Example 4 Rotomolded Bottles

Rotomolded bottles were manufactured using the following materials, which were ground into rotomolding powder (average particle size 300 μm) using as pulverizing system ULTRA Dual Mill from Reduction Engineering Scheer:

-   -   P130: prepared as described in Example 3.     -   P130b: prepared as described in Example 3.     -   P131: prepared as described in Example 3.     -   P131b: prepared as described in Example 3.     -   P132: prepared as described in Example 3.     -   P133: prepared as described in Example 3.     -   Lumicene® mPE M4043 UV from Total Refining & Chemicals available         in powder form: This metallocene-catalyzed monomodal         polyethylene has a density of 0.940 g/cm³ (ISO 1183) and melt         index MI2 of 4.0 g/10 min (ISO 1133 condition D under a load of         2.16 kg at 190° C.). The tensile modulus was around 650 MPa         measured according to ISO 527-1B.     -   Lumicene® mPE M4041 UV from Total Refining & Chemicals: This         metallocene-catalyzed monomodal polyethylene has a density of         0.940 g/cm³ (ISO 1183) and melt index MI2 of 4.0 g/10 min (ISO         1133 condition D under a load of 2.16 kg at 190° C.). The         polyethylene powders were obtained after grinding the         polyethylene pellets. The average particle size of the powder         after grinding was of 300 μm.     -   Lumicene® mPE M 3421 UV from Total Refining & Chemicals: This         metallocene-catalyzed polyethylene has a density of 0.935 g/cm³         (ISO 1183) and melt index MI2 of 2.7 g/10 min (ISO 1133         condition D under a load of 2.16 kg at 190° C.). The tensile         modulus was around 590 MPa measured according to ISO 527-1B. The         polyethylene powders were obtained after grinding the         polyethylene pellets. The average particle size of the powder         after grinding was of 300 μm.

The samples were evaluated on 7 L bottles prepared by rotomolding using a commercial rotomolding equipment. Peak Internal Air Temperature (PIAT) was 210° C. in all cases.

A carousel oven machine was used, with Teflon-coated Aluminum molds. Cooling of the molds was obtained via external air cooling only. Air was used at room temperature (between 20 to 25° C.). For each bottle, resin powder was loaded in the mold, followed by manufacturing the bottle by rotational molding.

The parameters of the cycle were the following:

-   -   Heating of the oven to a temperature of about 310° C.;     -   PIAT (peak internal air temperature): about 210° C.;     -   Thickness of bottles: 4.5 mm;     -   Removed from the oven at 200° C. (internal air temperature).

The densification (bubble removal) during the rotomolding cycle was assessed. Densification and bubble formation has been discussed by Kontopoulo et al. (M. Kontopoulo, E. Takacs, J. Vlachopoulos, Rotation, 28, Jan. 2000). During melting air pockets or bubbles are trapped, thus delaying the formation of a homogeneous melt and also affecting the aesthetical and/or mechanical properties of the finished product.

A charge-coupled device (CCD) camera was used to characterize the properties of polyethylene resin during densification. Characterization of the processing behavior was analyzed using a megapixel progressive scan interline CCD with on-chip circuits commercially available from Kodak. Its characteristics are given in Table 6.

TABLE 6 architecture: interline CCD, progressive scan, non-interlaced pixel count: 1000 (H) × 1000 (V) pixel size: 7.4 μm (H) × 7.4 μm (V) photosensitive area: 7.4 mm (H) × 7.4 mm (V) output sensitivity: 12 μV/electron saturation signal: 40,000 electrons dark noise: 40 electrons rms dark current (typical): <0.5 nA/cm² dynamic range: 60 dB quantum efficiency at 500, 540, 600 nm: 36%, 33%, 26% blooming suspension: 100X image lag: <10 electrons smear: <0.03% maximum data rate: 40 MHz/channel (2 channels) integrated vertical clock drivers integrated correlated double sampling (CDS) integrated electronic shutter driver

The high performance 15-bit (16 bits minus 1 bit for control) CCD sensor with transparent gate electrode provided 32,768 unsigned levels of gray, allowed the acquisition of about 10,000 frames/s and covers a broad spectrum of from 400 to 1000 nm.

The progressive disappearance of bubbles as a function of time and temperature is clearly and instantaneously followed. In addition to the visual aspect, the computer instantaneously produces a set of parameters resulting from picture analysis and allows to plot a graph as shown in FIG. 5. FIG. 5 shows the percentage of total picture area covered by bubbles (Aa, in %) as a function of the infra-red temperature of the sample (Tir, in ° C.). Using the polyethylene according to the invention, rotomolded articles with a low number of bubbles could be obtained.

3D shrinkage: The 3D shrinkage was evaluated following the method described in WO 2008/151988. 3D shrinkage was assessed using a mold of 7 liters in volume. This mold was used on a Ferry Carousel machines (Standard oven type). A 3D optical scanning was made of the inner surface of the mold. A 3D optical scan was made of the rotomolded article and a best fit calculation was done to compare the mold and the article. The rotomolded article and the mold have been scanned: Approximately 330000 points of comparison were carried out on both the article and the mold. A design of experiment (DOE) was performed to obtain the correlation between material, processing conditions and 3D volume properties of rotomolded articles. This DOE was used to calculate 3D “shrinkage”. The average 3D deviation was used calculate the 3D shrinkage with the equation: y(% shrinkage)=−1.2298103+−2.9087107×(Avg 3D deviation)

The results are presented in FIGS. 6 and 7. In FIG. 7, the abscissa x shows the difference in mm (deviation) between the mold and the molded article. the more important is the deviation, the more deformed is the article. The ordinate is given as arbitrary units (arb. units) and represent the number of points per class of deviation.

The impact properties of the rotomolded articles were measured using the method of standard test ISO 6603-2 at a temperature of −20° C. and −40° C. This is a falling weight test that gives the resistance to shock. The tests were carried out respectively at temperatures of −20° C. and −40° C. and a speed of the falling mass of 4.43 m/s. The test results were obtained on an average of at least 5 samples. Modes of failure during impact testing fall into two categories: brittle and ductile. With brittle failure, a crack initiates and propagates prior to any bulk yielding of the specimen and hence the point of failure lies on the initial rising portion of the load/deformation curve. In the case of ductile failure, considerable yielding takes place and the failure occurs well after the maximum on the load/deformation curve. As the area under the load/deformation curve is a measure of the fracture energy, it follows that brittle failure is associated with very low absorbed energy as compared to ductile failure. The ductility index is defined by the ratio Eprop/Etot, in % (i.e. (Eprop/Etot)*100), wherein total energy Etot is the sum of peak energy Epeak and propagation energy Eprop. The samples used for impact tests were all taken from the same side of each trial molding so that the results were made comparable to the molding conditions. They were cut with a bandsaw into squares of 65 mm×65 mm, the edges were cleaned of burrs and the thickness at the center of each sample was noted. The machine used was the CEAST Fractovis. The impact results recorded are reported in Table 7 and in FIG. 8.

TABLE 7 Energy to Peak Total Ductility Sample Temperature ° C. Load (J) energy (J) index (%) M4041UV −20 53.08 84.87 27.8 M4041UV −40 39.00 54.22 16.7 P 130 −20 49.22 77.81 28.1 P 130 −40 36.14 37.39 2.8 P 130b −20 46.27 76.28 29.8 P 130b −40 44.04 46.24 4.1 P 131 −20 35.39 45.33 13.3 P 131 −40 7.48 14.21 6.3 P 131b −20 50.53 83.65 32.4 P 131b −40 53.23 78.27 19.2 P 133 −20 16.40 24.24 31.5 P 133 −40 13.50 20.27 22.0

The tensile properties of the bottles were measured according to ASTM D638 at 23° C., 40° C. and 80° C., and deformation rate of 10⁻³ s⁻¹, using a video traction apparatus. The longitudinal and transversal deformations were measured; each essay was repeated 6 times. Table 8 depicts the results of this test.

TABLE 8 M3421UV P 130 M4041UV Temperature (° C.) 23 23 40 80 23 40 80 E (MPa) 760 1305 710 240 806 369 84 ν 0.4 0.437 0.466 0.453 0.442 0.443 0.445 Threshold (MPa) 4.00 6.45 3 1.4 4 2 1 Stress at break (MPa) >50 >40 >30 >20 >50 >20 >15 True deformation at break (%) >100 >100 >100 >100 >100 >60 >60

The next step consisted in determining the ideal thickness, weight of material to use in an unanchored cylindrical liquid storage tank with a height of 3677 mm and a diameter of 2340 mm, capable of resisting a maximal hydrostatic pressure of 0.0535 MPa when at 99% full capacity at room temperature (23° C.). The tests were conducted using Abacus® software, wherein the data from Table 8 were used to calculate the maximal Von Misses stress and the maximal displacement of the base of a tank (settlement) to assess the possibility of irreversible deformation (plastic deformation) of the tank. The modeling was performed on ¼ of the tank, since the tank was considered to be symmetrical on both the x and the y axes. The maximal displacement was on both the external (radial) and the internal (axial) surfaces of the tank; while the Von Misses stress was calculated at both the internal and external surfaces of the tank.

The results of the calculations performed with P 130 are depicted in Table 9. The results of the calculations performed with M4041UV are depicted in Table 10. The results of the calculations performed with M3421UV are depicted in Table 11.

TABLE 9 Maximal Maximal Von Mises Tank dimensions displacement stress (σ_(VM)) (MPa) Thickness Weight (mm) Internal External (mm) (Kg) Radial Axial surface surface 10 324.31 −5.67 −16.97 6.42^(p) 6.37 12 389.18 −4.67 −13.01 5.68 5.22 14 454.04 −3.97 −10.24 4.79 4.40 16 518.90 −3.45 −8.30 4.12 3.82 18 583.76 −3.05 −6.91 3.62 3.38 ^(p)Plastic deformation.

TABLE 10 Maximal Maximal Von Mises Tank dimensions displacement stress (σ_(VM)) (MPa) Thickness Weight (mm) Internal External (mm) (Kg) Radial Axial surface surface 12 387.53 −7.6 −20.61 5.67^(p) 5.26^(p) 14 452.12 −6.46 −16.53 4.83 4.42 16 516.70 −5.60 −13.49 4.16 3.83 18 581.29 −4.95 −11.25 3.65 3.39 20 645.88 −4.42 −9.57 3.25 3.03 22 710.47 −4.00 −8.28 2.92 2.74 ^(p)Plastic deformation.

TABLE 11 Maximal Maximal Von Mises Tank dimensions displacement stress (σ_(VM)) (MPa) Thickness Weight (mm) Internal External (mm) (Kg) Radial Axial surface surface 14 449.23 −7.17 −17.23 4.54^(p) 4.35^(p) 16 513.40 −5.95 −14.01 4.05^(p) 3.82 18 577.58 −5.26 −11.66 3.58 3.37 20 641.76 −4.69 −9.90 3.19 3.01 22 705.93 −4.25 −8.55 2.86 2.73 ^(p)Plastic deformation.

The calculated maximal Von Mises stress was then potted in function of the thickness of the tank, for both the internal (FIG. 9) and the external (FIG. 10) surfaces of the tank. The dotted lines on FIGS. 9 and 10 indicate the threshold for each of the polymeric materials. These figures show that a tank comprising a polyethylene resin according to the invention has the highest threshold.

The calculated radial maximal displacement was also potted in function of the thickness of the tank (FIG. 11). From figure is possible to determine that the tank comprising a polyethylene resin according to the invention is the less rigid. Table 13 shows the maximal displacement and maximal Von Mises stress for each of the tested samples.

TABLE 13 container Maximal Maximal Von Mises dimensions displacement stress (σ_(VM)) (MPa) Thickness Weight (mm) Internal External Sample (mm) (Kg) Radial Axial surface surface P 130 11 356.74 −5.11 −14.82 6.25 5.80 M4041UV 14 452.12 −6.46 −16.53 4.83 4.42 M3421UV 17 516.70 −5.59 −12.75 3.82 3.58 

The invention claimed is:
 1. A rotomolded article, comprising at least one layer: said at least one layer comprising: at least one metallocene-catalyzed polyethylene resin comprising at least two metallocene-catalyzed polyethylene fractions A and B, wherein the at least one metatllocene-catalyzed polyethylene resin has a bimodal molecular weight distribution, wherein each fraction of said at least one metallocene-catalyzed polyethylene resin is prepared in at least two slurry loop reactors connected in series; and at least one ionomer; wherein the at least one metallocene-catalyzed polyethylene resin comprises: at least 25% to at most 55% by weight of polyethylene fraction A based on the total weight of the at least one metallocene-catalyzed polyethylene resin, wherein fraction A has a density at least 0.005 g/cm³ higher than the density of the at least one metallocene-catalyzed polyethylene resin; and wherein the at least one metallocene-catalyzed polyethylene resin has a density of at least 0.930 g/cm³ to at most 0.954 g/cm³ as measured according to ASTM D-1505 at 23° C.; a melt index MI2 of at least 1.0 g/10 min to at most 25.0 g/10 min as determined according to ISO 1133, condition D, at 190° C. and under a load of 2.16 kg.
 2. The rotomolded article according to claim 1, wherein the at least one metallocene-catalyzed polyethylene resin has a molecular weight distribution M_(w)/M_(n) of at least 2.0 to at most 5.0, with M_(w) being the weight-average molecular weight and M_(n) being the number-average molecular weight.
 3. The rotomolded article according to claim 1, wherein fraction A of the at least one metallocene-catalyzed polyethylene resin has a density at least 0.010 g/cm³ higher than the density of the at least one metallocene-catalyzed resin.
 4. The rotomolded article according to claim 1, wherein polyethylene fraction A of the at least one metallocene-catalyzed polyethylene resin has a density as measured on a fluff of at least 0.950g/cm³.
 5. The rotomolded article according to claim 1, wherein polyethylene fraction A of the at least one metallocene-catalyzed polyethylene resin has a melt index M12 as measured on a fluff of at least 15.0 g/10 min.
 6. The rotomolded article according to claim 1, wherein the at least one metallocene-catalyzed polyethylene resin has a multimodal molecular weight distribution.
 7. The rotomolded article according to claim 1, wherein said at least one metallocene-catalyzed polyethylene resin is compounded with 0.1% to 20% by weight of at least one ionomer relative to the total weight of the compounded resin.
 8. The rotomolded article according to claim 1, wherein said at least one ionomer is a copolymer comprising one or more olefin or styrene monomers, and one or more monomers comprising a functional group selected from the group comprising carboxylate, sulfonate, and a carboxylic acid anhydride; or a grafted polyolefin or polystyrene comprising one or more functional groups selected from the group comprising carboxylate, sulfonate, a carboxylic acid anhydride, phosphonate, and phosphinate.
 9. The rotomolded article according to claim 1, wherein the article is a tank, a drum, a container, a bin, a vat, a jerrycan, a can, a cistern, a bottle, boat or part thereof, float, buoy, structural part.
 10. The rotomolded article according to claim 1, wherein the Temperature Rising Elution Fractionation (TREF) distribution curve of the at least one metallocene-catalyzed polyethylene resin comprises at least one peak appearing at a temperature of at least 97° C. to at most 105° C. and having an area under the curve of at least 5.0% to at most 25.0%; as determined using a crystallization rate of −0.5° C./min from 95° C. to 35° C., and column load volume of 1.90 ml.
 11. The rotomolded article according to claim 10, wherein the Temperature Rising Elution Fractionation (TREF) distribution curve of the at least one metallocene-catalyzed polyethylene resin further comprises two more peaks having the following features: peak 1 appearing at a temperature of at least 87.0° C. to at most 93.0° C. and having an area under the curve of at least 25.0% to at most 50.0%; peak 2 appearing at a temperature of at least 94.0° C. to at most 98.0° C. and having an area under the curve of at least 35.0% to at most 60.0%.
 12. A rotomolding process for preparing a rotomolded article according to claim 1, comprising the steps of: a) providing at least one metallocene-catalyzed polyethylene resin comprising at least two polyethylene fractions A and B, wherein the at least one metallocene-catalyzed polyethylene resin has a bimodal molecular weight distribution, wherein each fraction of said at least one metallocene-catalyzed polyethylene resin is prepared in at least two slurry loop reactors connected in series, and wherein the at least one metallocene-catalyzed polyethylene resin comprises: at least 25% to at most 55% by weight of polyethylene fraction A based on the total weight of the at least one metallocene-catalyzed polyethylene resin, wherein fraction A has a density at least 0.005 g/cm³ higher than the density of the at least one metallocene -catalyzed polyethylene resin; wherein the at least one metallocene-catalyzed polyethylene resin has a density of at least 0.930 g/cm³ to at most 0.954 g/cm³ as measured according to ASTM D-1505 at 23° C.; a melt index MI2 of at least 1.0 g/10 min to at most 25.0 g/10 min as determined according to ISO 1133, condition D, at 190° C. and under a load of 2.16 kg; b) compounding said at least one metallocene-catalyzed polyethylene resin with at least one ionomer; and c) rotomolding said at least one metallocene-catalyzed polyethylene resin compounded with said at least one ionomer, into at least one layer of an article.
 13. The rotomolding process according to claim 12, wherein said at least one metallocene-catalyzed polyethylene resin is prepared in at least two reactors connected in series. 